As of today, Steam now offers players refunds "for any reason," according to a new help page on the site.
The gist of it is this: Players can ask for refunds for nearly any type of content (games, in-game purchases, and DLC) purchased directly from Steam.
The only stipulations are that players requesting refunds must have purchased the content directly from Steam, and that, in the case of games, they must have played them for less than two hours. Refunds can only be requested within 14 days of an initial purchase. Pre-purchased games' two-week refund period begins at the day of release.
Notably, Valve says it's okay for players to ask for refunds on games that recently went on sale and subsequently re-buy the game at the sale price.
Users who "appear" to be "abusing refunds" are out of luck, says Valve: "we may stop offering them to you." Also, users who are banned from games by Valve's anti-cheat system aren't eligible to ask for refunds on the associated titles. (It's unclear if the same policy applies to users who are banned by the developers of the games, rather than VAC.)
For more details on the new refund system, hit up Valve's new policy page.