Here's an attempt to give basic guidelines for those who are looking to buy a new laptop, based on experience and study I did when I went to buy mine.
Laptops are usually more specific to personal needs, and are usually not suited to accommodate multiple needs, and also they have limited upgrade potential and options due to limited space available. Also laptops (unlike desktops) cannot be assembled after purchasing components separately, and all the components (specially processor, graphics card, modem etc) cannot be upgraded, rather in most of the models, only optical drives, hard disks and RAM can be upgraded, so choices are limited and hence, a model has to be carefully selected.
I divide the capabilities of laptops into three categories: Power, Battery (battery time) and Portability. Manufacturers offer different balance of Power, Battery, and Portability to suit particular needs of a customer, and all these affect the most important thing (the fourth category which, considered or not, just exist): the Cost, and it is almost impossible to get all three in one model, so a combination has to be decided according to the particular needs of the customer.
So, first, the customer needs to decide the budget, how much he wants to spend, and then work out what balance of power, battery life and portability needed, then shortlist the models which offer the selected or similar balance.
Laptops range from equivalent mid-range desktops to ultra-high spec laptops which match the most powerful desktops, but the power affect the other categories, battery life and cost, and sometimes portability. High specs: Fast processor, Graphics Card usually severely decrease the battery life and increase the cost, and a top range core2duo or core2quad based notebook with a gaming graphics card will make a laptop best suited for the desk with a power socket nearby, and going for low-specs specially a slow processor leaves the laptop capable of nothing except editing text documents, programming, using older versions of Photoshop, and asking for a replacement after a year or so, but usually gives a much longer battery life, and keep the cost low, and there are people who want to do nothing except edit documents, view and edit presentations, browse, listen to music etc. So a low spec laptop costing less with better battery life is an option for them, but most of the people aim for mid-spec, mid battery life and mid cost model, which in most of the situations, is the best option.
Processors in modern laptops range from Intel Atom for the low cost and ultra-small models to Intel Core 2 Quad Extreme for the extreme desktop replacement models. Intel Core2Duo and AMD Turion X2 lie in the mid-range and are usually preferred because of power to cost ratio. Intel Atom based models are extremely low cost and usually light weight and they are specially designed for extremely long battery life and ultra portability. Atom should be selected only if the budget is extremely limited or a Mini PC or MID (Mobile Internet Device such as Acer Aspire One, Sony Vaio P-Series, Asus Eee PC, HP Mini) is needed since it is the only processor used by most of the manufacturers. They are lightweight and cheap, and most cost less than $500. With up to 160GB Hard Disks, long battery time, small and portable construction, they are very useful as second and on-the-go PCs.
Intel Core2Duo and AMD Turion X2 offer the most flexible and wide range for many needs, they offer balance in performance and cost, and give great battery life. They are used in most of the mid or high spec laptops, including many gaming notebooks. Intel Core2Duo range offers 2.0GHz and 2.2GHz processors for mid-spec models to 2.53/2.66 GHz models, and 2.8/2.93/3.06GHz (T9600, T9800, and T9900 respectively) models which are preferred to Core2Quad and Quad Extremes in even top-end gaming models. Select a Core2Duo or AMD Turion Ultra and it'll be suitable for everything, giving adequate power and performance and will last for 2 years or more without having to upgrade. Cost and size of such laptops lie in manageable range, and hence are the most popular models. Most of the laptops with screens from 13.3 inches to 17 inches use these processors, and they are also available as option in most of the expensive models. HP, Dell, Acer, Sony Vaio, Toshiba, and most other manufacturers offer the widest range of models with these processors for buyers looking for laptops for wide range of applications. They may cost less than 500 USD (Compaq Presario, HP G60t, Dell Inspiron, Toshiba Satellite L300 & L500, Sony Vaio, and many others), and are flexible and manageable. The portability and power are well balanced, and with the latest 45nm technology, there processors are extremely efficient. For budget buyers to mid-high range buyers, these processors offer a great balance of cost, power, portability and efficiency.
Core2Extreme, Core2Quad and Core2 Extreme Quad are the most powerful processors available in laptops today, they almost match the fastest desktop processors, and offer extremely high performance needed for gaming, high-end video editing, and CAD. Models offering them are expensive, usually not portable (17" screen) and generate a lot of heat and are power hungry, so they should only be preferred if such power is needed. Core2Quad QX9300 is the most powerful Mobile Core2Quad Extreme processor, and it scores 3677 in 3dMark06. Core2Quad Q9100 and Q9000 complete the range of Mobile Core2Quad processors. Many models like HP Pavilion DV7, HP HDX 18t Premium, Toshiba Qosimo X305-Q708, Alienware M17x are some models that feature Core2Quad and Core2 Extreme Quad processors. They usually cost more than $2000, are bulky (16 inch or larger screen) and heavy, so portability is not good.
Mobile Core i7 processors are now available and are being used in top end laptops. Core i7 920XM (2.0 GHz), Core i7 820QM (1.73 GHz) and Core i7 720QM (1.6GHz) are the processors available in the range. They are among the most powerful laptop processors available and laptops based on them are pretty expensive, but they should be cheaper than laptops using desktop processors. And since they are mobile processors, laptops using them are more portable and efficient, and Core i7 based laptops are getting smaller and cheaper, many have 15 inch screen and weight equal to a mid range Core2Duo based laptops, and cost much lower than the extreme machines. Mobile Core i7s have extreme efficiency and power. Many models use these processors now, like Alienware m-15x, Dell Studio XPS16, MSI GT740, MSI GT640. And many like Dell Studio 16, MSI GT640 have size, weight and cost in the mid range.
To fill the little performance gap between the top-end laptop and desktop processors, a few laptops also offer desktop processors, and desktop processors commonly used in laptops include the common desktop processors like Q6600, Q9550, E8500, E6800, E6850 etc and most of the time, processors of such laptops can be upgraded as well which is an added feature, which most of the other laptops lack. Hypersonic Aviator EQ7 Ultimate (Core2Quad Q6600), Puget Systems D7800i (Core2Duo E8500) are 2 examples, and both cost more than 4000 USD, and weigh over 4 kilos. Such laptops offer some of the best performance, specially the ones with Q6600 or QX6850; the QX6850 beats the best laptop processors by a significant difference, and scores 4467 in 3dMark06 CPU. But they are again the extremes, in size, cost and power consumption. Desktops with such top-end processors are very expensive and a few people buy them, so they increase the cost of laptops as well, expect to pay above $3500 for such model.
Core i7 based laptops using desktop processors like the Core i7 920 and Core i7 Extreme 975 are great. Core i7 920 scores 4950 in 3dMark06 and beats every processor that is used in laptops. But the drawbacks are again costs, size and consumption. SmoothCreations X58 i7 (Core i7 920, 940, 950, 975) is an example.
Laptops today have DDR2 and DDR3 RAM and usually the standard models can accept a maximum of 8GB RAM (2 x 4GB) and adding another module or using larger capacity modules have a negligible effect on batter life, so here, the larger the better. 2GB RAM is enough for most needs, but will require an upgrade after a year or so, and adding another 2 GB module is pretty cheap so 3 or 4GB is appropriate today. Core2Quad and Core i7 based laptops now use DDR3 RAM which is much faster, but majority still uses the DDR2 RAM which is a lot cheaper.
Hard disks in laptops are available from 160GB to 500GB models (5400 or 7200 rpm), and most 15 inch models accept one Hard Disk (a few, like Alienware Area-51 m15x accept another Hard Disk in the Smart Bay), whereas larger models usually have room for 2 and sometimes even 3 Hard Disks. There are no specific guidelines for selecting the HDD space, it depends on what the laptop will be used for, and usually the best option is to select the maximum affordable size. Selecting a larger hard disk have a negligible effect on battery life and adds almost no extra weight to the machine, but adding another hard disk adds considerable weight and requires more power. It is also quite costly. External Hard Disk is always an option. Some high performance laptops have Solid State Disk (SSD) instead of a hard disk which is pretty expensive but is a lot faster and efficient.
Graphics Card does not add a lot of weight, but it does add a lot to the cost and consumes a lot of power, so a high-end card should only be selected if the machine will be used for gaming, watching HD movies or video-editing, for all the other needs, an on-board model is more than enough.
But if the laptop will be used for gaming, there is a wide range available, including GeForce 9200M to the mighty GTX 280M (3dMark06 score: 11767) in the nVidia range, and ATi Mobility Radeon HD4870 (3dMark06 score: 10183) in the ATi Range. But the most suitable card for mid-range models is the 9600M GT (3dMark06 score: 5154) or 9700M GTS (3dMark06 score: 7779) (512Mb or 1GB Models) which gives adequate gaming performance while keeping the cost, power consumption and heat in the acceptable range. They work well with the latest games offering decent FPS and resolution and may work with next-year's games as well.
But there are extremes as well, laptops offering Dual Graphics Cards (SLI or CrossFire), such as GeForce 9800M GT SLI (3dMark06 score: 14000), GeForce GTX 280M SLI (3dMark06 score: 15000), ATi Mobility Radeon HD4870 x2 (3dMark06 score: 14120) are the extremes, whereas GeForce 8800M GTX SLI (3dMark06 score: 12184) is a good choice, but they are offered in huge laptops which weigh over 4 KG and are really expensive. Very few 15 inch models have dual graphics card option. Specialized models for gaming include Dell XPS, Alienware M15x, Alienware M17x, Asus G series, MSI GT740 and MSI GT640 Series. They offer the performance needed for latest games and most are enthusiast-only models due to their cost and size.
Battery time of laptops is very sensitive to many things, specs, screen size, no of HDDs, graphics card etc. Most standard 6-9 cell batteries are good enough; they give around 1:30 to 2:30 min with usual load and 45-50 min on maximum load (gaming etc). Battery time of a model is usually given by the manufacturer, and it is quite different sometimes in models from different manufacturers with similar specs. In most of the models, battery can be upgraded, and it is always possible to purchase an extra battery to carry as spare. Larger battery (12 cell battery used in many models) adds weight and adds to cost but it is usually cheaper and more convenient than 2 batteries. But regardless of the battery size and cost, it more depends on specs, so a wise choice should me made if the laptop spends a lot of time off the desk. Models with particularly high battery time are available and they can also be considered.
Size and Portability
Size adds weight, this is simple, extra batteries and HDD also add weight, but size is the main factor, and size varies from manufacturer to manufacturer but it mostly depends on screen size and hence, it also impacts cost.
Screen size ranges from 8.9 inches to 20.1 inches, but in my experience, 15.4 inch (usually written as 15 inch) is the standard size, and selecting a smaller or larger size with same specs increases the cost. Usually 1 inch adds 500g to 750g to the weight of the laptop, and makes it more inconvenient to carry around. Screen consumes most power in a laptop, so larger screen consumes much more power, and decreases portability, but at the same time larger screen adds all the benefits of a larger screen, ideal for graphics designing etc. Larger screen models also have more space, so they usually offer 2 or 3 Hard Disks, dual graphics cards and other options.
On the other hand laptops with small screens (MIDs or Mini PCs) are ultra light and ultra portable, but have limited power and performance. The best choice is 15.4 inch or 14.1 inch models which offer almost all the features and keep the weight manageable. 13.3 inch models are considered ideal for students, but are usually pretty expensive.
Resolution is pretty much the same; most 15 inch models have a maximum resolution of 1280 x 800 pixels, whereas 1440 x 900 or 1680 x 1050 is offered as a cost option. So, higher resolution which is standard on larger screens is also available on smaller screens, even on 13.3 inch models. Some manufacturers even offers 1920 x 1200 (1200p) in 15.4 inch models. Now laptops with aspect ratio 16:9 are becoming more popular.
Other items include Optical drives, WI-FI, Blue tooth and web cam.
CD / DVD ROMs or Burners are available, and are pretty cheap. An optical drive can be replaced later. DVD Burners are pretty common and found in most of the laptops in the market and now Blu-Ray ROMs are also found on mid-range models, which are good for future proofing, but Blu-Ray writers are still pretty expensive, and very few models have them even as a cost option.
Wi-Fi, LAN and Web cams
These are pretty common on all models; rather it is difficult to find a laptop without these, whereas blue tooth is a cost option on some models but is standard on most of the others.
Processor benchmarks list: http://www.notebookcheck.net/Mobile-Processors-Benchmarklist.2436.0.html
Graphics Cards benchmarks list: http://www.notebookcheck.net/Mobile-Graphics-Cards-Benchmark-List.844.0.html