#1. You can't thank yourself.
#2. There is a daily limit of ten, so you can't over do it.
#3. Certain forums have it turned off.
#4. After you thank someone, the button expires. You cannot thank them more than once.
#5. Yes, you do have to be logged in.
#6. You must have made one post before the thanks button is available.
#7. If a post is older than 30 days, thanks is not available.
The deal is that we don't want someone to attempt to thank EVERY post on the board. (It's been tried.)