Teeny-weeny memail pony: put the delete button at the top and bottom of the list - since newest mail is at the top, it is more likely to get deleted.
My Little Favorite Pony - for marking posts and comments as favorites, how about allowing you to put in a little note as to why it piqued your interest.
My Favorites Pony - Love the favorites. How about adding into the contacts section a "jump to contacts' favorites" link?
Ask MeFi Karma - I think it would be cool that if you are reading the comments of an AskMeFi thread that you created, you could rate an answer by granting or taking away a karma point from the answerer. In this way, there will be a new mechanism
of MeFi elitism to encourage constructive replies.
Please oh please make it possible to view all of a given user's posted topics. Perhaps a link to the link on the user's profile page?