It would be nice to see what users have recently been active. This is somewhat standard with many forum-style websites, showing how active the site has been in the last n-minutes or so. As a functional advantage, this also serves to let you know when is a good time to get a question in.
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Why are there so many request centered toward "user activity"?
I think stackoverflow (ant its family) is more Q&A centric, with no particular value (other than maybe curiosity) to the user asking or answering (well, in these case the reputation of the answerer may be important, but only in the context of the answer, I don't think it has value to want to follow every answer by one user)
Anyway, it's my opinion