This is a really terrible idea. Aside from giving people karma for really bad reasons (political discussions, spammy off-topic stuff, etc.), it's just going to lead to a lot more unpleasantness. Some of the threads in Off Topic are bad enough, but would be much worse if you involved additional people through Thanks! that would otherwise not have commented.

I know you have a big think for polls and such, penguinbait, but Thanks! in Off Topic sounds like a really good way to create wide rifts in the community.