With all the drama, butthurtbronies, and wannabe trolls on this site, the reputation system would be a retarded idea.
There's so many cliques on here and unbeknownst drama that people would purposely down vote someone's "karma" or do it just because they don't like them.
Bronies being given bad karma for being bronies. Wannabe trolls would have their alts or fellow trolls down vote others or up vote each other.
It's an interesting idea and I honestly do like it. But looking at the reality aspect of it, it wouldn't really work.