Then a bunch of noobs will be like "LOL LETS KEEP GIVING EACHOTHER AESOME RATINGS" and the whole thing breaks.
If there is a way to break it, you can be sure thousands of morons will spend their days breaking it.
There are some counter measures for that, for example, PlayerA can give PlayerB a rating only once per month.
And when a player gets to many positive ratings one after another in a short amount of time the system will mark him for inspection, resulting in punishment in most cases.
Ah, I'll make it so you can only give 10 ratings each day.
And by the way, it will only be two ratings possible, good or bad.
Maybe one rating per day ?
yeah, comparable above.