It is a bad idea.

Giving "Thanks" shouldn't have any influence on karma.
If everything you do is going to be ranked then this forum will be turned into some kind of childish game.
Making an indicator of "Healthy account" and link it with giving thanks might be a good idea. (Many people don't like to post, but giving thanks will reflect a good healthy account in their profile[in future there might be a use for this])


Just think of this:
@Texrat ,zerojay and others: Wanna swap "Thanks"? (We all are going to get double Karma)

For those who are interested, you can easily pm me.


Anyway, this might be the worst example to give however, if someone is going to suggest an idea to rank a bad post (Some thing that shows how ungrateful or being thankless). You will get my vote for it (This will get me to change my signature into something more useful). ;]