At one of my past jobs we did "karma points", with similar discussions around how people would game the system to earn more points. To limit that, we set a limited number of points into the system...

Was a software development team, and at the start of each two week sprint/iteration each developer was given 10 points that they could then use to "Thumbs Up/Down" anyone else on the team. When you'd used up all of your points, that was it, you had to wait until the sprint/iteration was over before you could get any more to use to Thumbs Up/Down someone. You didn't get to keep your points and carry them over to the next 2wk period either, so people didn't end up with ever-growing piles of points that they could then use to Up/Down someone to death.

For us, worked fairly well. At the start we still had a few team members that wore through their points super-fast and were "broke" for most of the iteration and couldn't up/down anyone, but that settled out after a few runs and people became more cautious about how they spent their points.

On a somewhat related note.... is there a way to do a search for or view a list of the "most thumbed up posts/threads" in the system?

If you tie the number of coins you receive back to activity, IMO that just opens it up again for easy abuse. But yes, my experience has been that by limiting the supply of coins to the system, people start to get more picky about how they spend them and it then becomes more special to receive one (which is the point, isn't it?)