Valve recently updated Steam to integrate with their new refund policy, which is much more forgiving and fair, letting you return games within a larger window, even if you play them for a small amount of time.
The new Steam return policy allows you to return games within 14 days so long as you've played no more than 2 hours. DLC can be returned so long as it hasn't been used at all.
Steam has a new page dedicated to their new refund policy and has outlined the new rules that make for a far more fair refund policy than previously. Perhaps this is in response to the BBC investigation into the PSN's rather shady refund policy of their own. Regardless, this is a step in the right direction.
Steam will allow you to be refunded within 14 days and no more than 2 hours of actual gameplay. For pre-purchase games the clock starts when it's actually released and not when you made the purchase. Of course you can request a refund at any time before it's available as well.
DLC is refundable if the base game hasn't been played for more than 2 hours and if the DLC hasn't been consumed or used in any way. In-game purchases from Valve developed games can also be refunded within 48 hours if it hasn't been used. Third-part developers can opt to use that policy themselves, but aren't required to by any means. Valve says that it'll be clearly mentioned on the games page.
Requesting a refund is easy, just go to the support page, click on "A Purchase," click on the game in question and select the "It's not what I expected" selection. It'll tell you if you're within the window and even if you aren't, you can request your refund and leave a little note or explanation if you'd like to. It's really quite simple. Oh yes, and this is important, if you've been VAC banned from a game, then you're ineligible for a refund. Cheaters never prosper in this case. And don't be abusing the refund policy for sale prices either, they might not refund you at all in that case. And then they might just take it away from everyone. Don't ruin it for the rest of us please.
Refunds are designed to remove the risk from purchasing titles on Steam—not as a way to get free games. If it appears to us that you are abusing refunds, we may stop offering them to you. We do not consider it abuse to request a refund on a title that was purchased just before a sale and then immediately rebuying that title for the sale price.
Steam's new refund policy is far more fair than it used to be, and allows one to make sure that they actually enjoy the game. Before you were stuck with a game if you played it for any amount of time. As soon as it's started, you're stuck. The new policy that's gone into effect allows you to try before you truly commit to anything.