If you “admire” the system, go for it.   But consider that I have encountered many discussions berating other forums for including downvotes and the ability to “lose” reputation.   Ratings systems can produce anti-social behavior and may drive users away.   Consider, for example:   rating posts but not people; allowing only upvotes; tallying up and down votes separately; counting only positive votes against a user’s reputation (if you choose to track it at the user-level at all).   Consider carefully the social impact of the system that you implement, because it will have an impact, and many users do not tolerate negativity especially when directed personally at them.   The implementation itself is quite trivial, requiring only one (or two) integer counter(s) in both the user and the post/reply record.

If you do implement this, also please provide the ability to search for posts based on, say, minimum number of upvotes.   (Another reason to count ups and downs distinct from one another.)   Where forums provide this capability, I use it a lot ... but I only want to see how many people liked the post or topic.

Bonus points if you implement “rate this post” and/or “rate this thread” with one-or-more individual criteria ... and double bonus-points if we can search on that, too.   As a forum matures and begins to carry hundreds of thousands of entries in its database, “finding(!) the good ’uns” can use all the search-options it can get.   Let there be participant-rating of content, but then, let me do something useful-to-me with this data, like refining my search.   Go further than PerlMonks has done.