Ok, now this is really cool. Steam just started a return policy.
Steam Refunds
The gist: You can return any game you have purchased on Steam for a full refund within two weeks of purchase (for any reason), as long as you have less than 2 hours of gameplay logged. Same goes for DLC, as well.
More details at the link above.
Important note, though:
So, I guess using the service as a way to play a bunch of 2-hour demos is frowned upon.
Steam Refunds
The gist: You can return any game you have purchased on Steam for a full refund within two weeks of purchase (for any reason), as long as you have less than 2 hours of gameplay logged. Same goes for DLC, as well.

Important note, though:
Abuse
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.
So, I guess using the service as a way to play a bunch of 2-hour demos is frowned upon.