I'd like to suggest that the rate of change of a user's reputation be displayed with profiles and signatures. This would be the 1st derivative to the line in the user's reputation graph.
This would encourage new members to participate more and would also show their potential. Since SO has been going for some years, from the point of view of a newcomer it is impossible to ever reach the top ranks in terms of reputation, but being crowned as a "fast riser" could be a nice motivational prize.
Just my 2c. :-)