I’m not sure you need it. You seem to have a pretty good base of commenters, though of course, given your subject matter, I’m sure there’s some pretty awful stuff that doesn’t make it through moderation.

Personally I can’t stand the karma system. It seems to work well enough at Slashdot, but at Plastic, a more politically oriented site which started very promisingly in 2000 or so using a similar karma system, cliques developed and gangs of partisans began rating comments by their ideological opponents downward without regard to whether the comment was in in fact (+1, insightful) or (-1, stupid).

In the end, Plastic ate its own tail. It still exists but has very few readers, which is a shame because it launched with support from the late, lamented suck.com, and had a great userbase at the start.

I suspect that politically (or legally) oriented sites can’t succeed if they allow the mob to decide what’s clever and what isn’t. The temptation to hit “-1″ when one disagrees is too great for some people to be allowed to exercise it.