This is just a wild idea without much thought, but I used it on an old web site I maintained that had social interactions. Perhaps you could make a similar system work by giving a salary of points to members. The longer they remain a member, the more points they get, but at the same time, you could make it so a portion of their points are transferred when they vote. This might encourage point hoarding, though, which was a problem that existed with my prior system. (In actuality, my system gave points for participation (visiting the site with some frequency) and removed points through a 'Karma tax' for not visiting the site, and then people could transfer points to each other.)
We use a system that is very close to what you describe on a very large site that I have helped to build and it actually works really well.
We're thinking of ways to allow people to turn their karma in to more tangible forms of rewards as well, such as one-shot functionality that you pay for with karma and physical goodies from a 'karma store'.
There was a similar thread on here regarding abuses of downvoting. One idea was to do the following:
1) Earn points by commenting and submitting.
2) Earn points by being voted up.
3) Spend points by voting someone down.
4) Loose points by being voted down.
