Because I use Firestorm, I wasn't aware that the official viewer's inventory didn't have cut-and-paste. I don't know how people can get along without it!
In any case, one feature that would be lovely to have is to be able to put on prim items as easily as you can other types of clothes. I mean, if an item is highlighted, and you hit ENTER, it should go onto your body, whether it's a prim or not. Ditto for double-clicking an item. It would make inventory far more intuitive, since all items would behave the same.