The old system with the Thanks button has now been replaced by a link with the word Like. It serves the same purpose, so you can use it to thank someone or show agreement, etc, but has extra features- a thumb appears a the top of the page, which allow you to find popular posts in longer threads.
The rather excitable makers say that:
The "Likes" system allows your users to effortlessly provide feedback to one another on the content they find most useful. Most importantly, this activity creates an underlying layer of data that works as a "Social Graph" for your community, opening the doors to a myriad of USEFUL features that have the potential to alter the way we interact with forums in different ways.