Right now, collection management is extremely barebones. I can rename or delete collections, delete games from collections, and I can change the display order of games. So far, this is it.
It would be nice to have the ability to more easily organize collections. Changing the order of games by drag/drop rather than repeatedly clicking left/right buttons to shuffle things around one by one. The ability to move games from one collection to another. The ability for one game to occupy multiple collections. The ability to merge collections. The ability to select a set of games from one collection and create a new collections with those games in it.
This all seems like pretty basic stuff to me.