Hey,
I'm not sure whether this has been suggested before.
I find "thanks" or "like" button will be useful because:
1. It shows appreciation and encourages people to help. Of course the help itself feels good but everyone would feel happier knowing their help is appreciated.
2. A majority of helpful posts are not qualified for a rep
3. Posting a new reply just for thanking is more abusive and spamming to the forum than simply clicking "thanks button".
Let me know what you guys think.
Once you reach 50 posts you will have access to a +1 button that you can use to applaud a helpful or quality post. The reason it takes that many posts to reach is because the system (and it's inverse, the -1 smite) were being misused and abused by a lot of new people to the forum. In the meantime, before you get there, there is absolutely nothing wrong with replying "thank you" to people you feel have been helpful. :)