Your wanted list can be matched up with another users for trade list and can be used to identify other users with whom you might want to trade games.
The wishlist has a feature where you can rank your desire to aquire a game. It also has a feature that allows you to email a link to your wishlist. It's purpose is to allow friends and loved ones to see which games might good holiday or birthday presents.
That's the intended usuage. I'm sure that each individual user may use one or both lists in different ways.