One of the not so obvious problems with Steam (which I love and use regularly) is that all your eggs are in one basket. If, for whatever reason, Steam decides they're going to cut you off then you'll lose access to all of your games.

A real world example of this is a friend who purchased a Steam game which did not work on his computer (unrelatedly due to overzealous DRM). Steam would not refund his purchase so he threatened to file a credit card chargeback and they replied that their policy in the event of a chargeback is to disable all of the users games immediately. Because he had >$300 tied up in Steam he wasn't about to lose it all over a $60 charge.