Scammers have made a habit on Steam of purchasing games in bulk, getting people to trade for them, and then flip their proverbial finger as those people's faces turn red with rage when their access to the game is revoked because the original payment was denied.
Thus, Steam has changed its rules on gifts, so that when it's placed in the purchaser's inventory, it remains untradable for 30 days. You can still gift games at any time, but trading can't be done.
Some commenters would also like to see a system with trader trust, similar to seller ratings on eBay and the like. Others are livid enough that they've started a Change.org petition claiming that this is just a way for Steam support "not being responsible for reversing trades involved with revoked games due to credit card chargebacks / fraud."