I have been noticing that more and more user's image icons are getting drop shadows behind them. Is this a display option or some sort of merit based achievement?
When you see a new feature, always check the StackOverflow blog: http://blog.stackoverflow.com/2011/09/expanding-user-cards/
We treat the expanding user card as a bit of a privilege, so you must have 1,000 or more reputation for it to appear. Any user with less than 1k rep will not have a user card hover by definition. And, of course, there has to be a reasonably complete user profile, otherwise there’s nothing to show!