It sometimes bothers me to see users who ask questions and gain reputation, but who otherwise do not participate at all.
His accept rate approaches 0 and he never cast a single vote, neither up, nor down. In other words: He lurks (which is absolutely okay) and leeches. This would be okay on paid forums or forums where there is no rep-system, but that's, AFAIK, not how the whole Stack Overflow -Buzz and -Community works.
An idea that came to my mind is a metric similar to the acceptance rate, or even a generalization of it, such as:
participation-rate <- max(1, votes-cast / (x*questions-asked)) , with x>0 & x<=1
This could then be displayed next to a user's acceptance rate.
Jeff gave an answer to which I totally agree, but on the other hand, I think his opinion also holds true for acceptance rate: Users who don't accept do no harm, too, but still that was something voted in for the sake of reputation-interested users.
I removed some ideas that are bad for potentially good content in itself. I also request the locking of this question as I come to the conclusion that such a metric is probably too fuzzy.