If people like a thread then surely they can take the time to type a thank you/like message of ten letters or longer...
Britishblades had a rep system and that went horribly wrong at one stage...
It did, any like/rep system it is prone to abuse by cliquey mates. The rep system has been reintroduced on BB and is working well and as intended so far. As soon as it doesn't it'll be dumped.
On a forum such as this with more traffic and groups of good mates, I don't think it's a good idea. Just take the time to post your approval, or disapproval. A forum is about speaking, not clicking a like button.