I may be wrong but I could have sworn we used to have a thank you button a looooooong time ago.

I say this because there are posts I like but because I have a habit of just clicking my recent posts to see when my shows I'm watching get updated, so if I don't actually have anything to contribute (the exception to this rule is when I am in a manic phase), but thanking people for shows they have shared when there is a post length requirement is kind of a ... ... deterrent. (actually I think that's the main reason which far supersedes (sp?) the my posts list, which actually is more of a passive complaint... or an excuse if you will). But I think a thank you button would be quite nice for when I want to let people know that I appreciate what they are doing, without making a couple hundred (or thousand even) posts that say "Thank you for the upload."

Now if there is a reason for not having it...
Resources- perfectly understandable
abuse- you don't have to make the thank yous actually benefit you in anyway and if anything you can make it private between the giver and receiver (I would think).

I honestly am probably one of the few people that loves the thank you button and am sure most people would overlook it, but I would just like it so I can give my thanks and still be lazy.