Dunno if this thread is still active, however I think that a good way to lure more users is to offer a truly unique feature.
An idea should be the open trade offer.
When I propose a trade, I don't really care who is the person that close that deal (save for a minimal rep requirement).
A nice feature should be to make an open offer, where I state the game I offer and a list of games I'd like to trade it for, and publish it in a sort of open market.
This should be coupled with the possibility to select one of my game and see what I can get out of it (in the open market), with the possibility to create a quick offer if something catches my eye.
Server-side, all the open offers and the searches should be matched automatically (using hash tables pretty much as the normal matching is done).
I think that most of the procedures required are, in face, already implemented in the project.
Correct me if i'm wrong, but I have never seen such feature anywhere, and it could really came handy, especially because I can discover games - potentially out of my wishlist - that are 1-click away to be traded.