I was looking at a question today and I noticed that the OP and I have drop shadows beneath out avatars and the person who edited the question and the other answerer do not.
Who gets a drop shadow, and why?
The avatars with a drop shadow are avatars that show a popup when you hover your mouse over them. For this to happens, the user must have entered something in the "About Me" field, and have a reputation higher than 1000. There could be a minimum length for the "About Me" description: When the length of the entered text doesn't reach that limit, the popup is not shown.