Measuring karma was a heavily-debated topic for a while on HackerNews. The goal is to provide some measurement that both accurately measures overall contribution to the community and encourages consistent engagement. Several solutions were discussed and a few were even tried. For example, pg tried to replace the overall karma score with an average score. All three combinations (total only, average only, and both) were juggled around in the top right corner, until eventually the simple total was used.
While all these attempts were good ideas, I think there is an even better metric that should be used here: the Sharpe ratio.