Stackexchange kind of does this. You need a certain (low) amount of karma before you can post certain types of submissions.
I think it's important to let new users have a visible voice, but giving older users greater powers for moderation might help preserve the older attitude of the site.
Popular comments will make more karma than they cost, so users will still be encouraged to leave comments that will become popular.
It seems that a system like this will be even more sensitive to what community considers popular. For this to work well you'll need to make sure that comment being popular correlates with it being good. To improve on that you'd may need to further reduce inefficiencies (e.g. time-of-day vs popularity) and maybe implement un-democratic measures if "voice of the community" still doesn't correlate with good.
I'd split test this system (and any other change like this). Have some posts that have these new rules in place (this should be publicly visible) and some that don't. See how this affects the results.
This would probably work well if combined with the private messaging function mentioned elsewhere on the thread.
I disagree. I think it would lead to a mix of bland groupthink and fashionable rebellion, with no room inbetween for the merely thoughtful.
I think in general you already have most of a filter in that bad comments get pushed down and the lower sections of comments seem to be read much less often.
Why is seniority good in and of itself? Privileging seniority seems anti-democratic and anti-newbie.
A karma penalty is a very soft slap on the wrist, at best.
It won't stop trolls, or assholes, or anyone with an agenda.
Subtracting a fixed amount of karma also gets less effective the more karma one has accumulated. So people with a lot of karma will be able to get away with more than people with less karma.
This has an upside, in that it allows more valued members of the community to express themselves more freely. The downside is that they can act like assholes without much repercussion. If the community winds up rewarding them for acting like assholes (by, say, upvoting their assholish comment) then that's even worse.
Of course, any community that not only tolerates but encourages assholes is not a community I want to be part of, and I've left a number of communities over that sort of behavior.
But there are other solutions short of "love it or leave it". I describe one such alternative in another comment in this thread: