Unlimited reps allow you to rep someone more than once in a row (But not for the same post if I recall). I think it also bypasses the limit to how many times you can rep people in 24 hours (I think you can only rep 5 people maybe?)
On the downside, your rep power goes waaaaay down when you enable unlimited reps. As a result, if you want/have to neg someone, they will barely feel the pain. Thats why I turned unlimited rep off myself. When a troll needs to be negged, they need to be negged!