We just implemented another highly requested feature -- the ability to mark a question as a favorite. You might have already noticed it by now, as every question has a large, clickable favorite star just under the voting arrows.
Click to favorite. Click again to un-favorite. Once favorited, these questions will show up in the "Favorites" tab on your user profile page. The default order is last modified, so favorite questions with the most recent edits or answers will shuffle to the top.
I wasn't totally convinced that marking something "favorite" was conceptually all that different from voting it up, but repeated and persistent requests on UserVoice eventually convinced me.
By the way, if you're wondering what sorts of things we are implementing in Stack Overflow on a daily basis, I strongly suggest checking out the Completed Tab on UserVoice. We always go through and mark all related requests (even the declined ones) as completed when we implement features that we believe satisfy those requests.
To encourage adoption of this new feature, I'll probably be adding a few badges around favorites soon.