I personally don't like this. If they really wanted to say thanks they would send them a PM or leave a post. It also makes the forum look untidy in my opinion.
Good idea, though.
We have considered adding this in the past and is on our to-do list. But due to all these disturbances, we've not had time to ponder over it. Also, we're trying to add a CSS Button for it.
Whether it be Alex's thread or MCG's thread, I think this is a good suggestion. Hope they do get around to adding it after all the disturbances (that's what Jimmy called it).
This has been brought up many times before, and I see some problems to be honest. It could just be like the thumb up and down system if we don't add regulations such as use for upgraded members only (could turn into everybody just thanking everything that isn't necessary).