I was thinking of LinkedIn and their recommendation system. This is a different website of course but I was thinking that having the possibility to leave a "thank you [...]" that would stay written in hard stone to users who really helped us would be great.
I don't think that would add any real value to StackOverflow. It would have no place in the interface. Would clutter user profiles, AND would add one more source of spam.
I think user would use that to relieve their frustration against somebody who closed their answer or whatever else he did to them. That would be one more source of text to "moderate", and one unnecessary source of text to moderate.
And after all, what would it add? I mean OK, I could see I helped people... but I can already see that by having accepted answers / upvotes / bounties / comments. Do we really need a wall for this?