Many times a member wants to thank another member for a useful post but really don't want to post a reply. Many also DO reply just to say "thanks".
Now you will notice a "Thanks" button near where you would hit reply on everyone else's posts. When you do so, it will appear underneath that you thanked the user for that post. Also, in the users profile in the "statistics" section it will show how many times they have been thanked - as well as in their postbit section just above their reputation.
Let me know your thoughts on this addition and whether or not it's useful, as I can easily remove it if you don't find it handy.
Look at this thread for an example where I thanked a member:
http://www.debatepolicy.com/showthread.php?t=27701