Keeping track of who "liked" who, when each "like" was given, which particular reply "triggered" the "like" and a comment for each "like", plus displaying a list of all "likes" in user's profile page is not as simple as keeping a "karma counter".
Implementing all this might not a big deal either, but given the number of users, messages and posts in this forum I guess even a small increase in php and sql load could add significantly to the overall server load.
Just thinking out loud... 