Likes are an important part of the community values and users "like" to see popular threads, based on other user opinions. It will definitely help the admins to evaluate better the content of their forums.
The feature will show how "likable" each thread is, when viewing the forumdisplay page.
It should be also a choice for sorting the list of threads in a forum.
IMO, the best non-intrusive location for thread likes would be in the lower right corner of the popup that shows when you hover over the thread title. Users will have the choice to edit their templates and make it visible into thread row, if they really want to display the number of likes on a more visible way.
Just thinking out loud, you can actually define how "hot" is a thread, based on the total number of likes. Many nice possibilities with this feature.