In some ways, this seems analogous to Stack's reputation system. Any thoughts on taking that similarity further? The two things that seem particularly worth consideration are unlocking site privileges -- I'm not sure we really have any need for that, but it's there if we do -- and, far more interesting IMO, the bounty system.
Also seems worth digging up something I mentioned a while back: the Unity Feedback system has a really nice way of minimizing brigading. There, each user has a fixed number of votes for endorsing feature requests. You can put all of them on one feature, split them across several, or hold on to some; and you can shift them around any time you want. I don't think the Thank You system should be limited that way, but it seems like a good model for a "kudos" system where individuals can be recognized for great contributions across the site. Rather than a fixed number, users could earn the right to give these based on their own contributions (e.g. one at 1000 karma, one for getting 10 thank yous, etc.) and while they wouldn't have any inherent meaning attached, it seems like a really good way for a community to identify its own experts.