Apparently you need 50 posts to be allowed to thumb-up or thumb-down.
That is correct.
It helps prevent people from signing up for a new account during the heat of an argument to downthumb their opponent. Those 50 posts are like the 3- day cooling off period when buying a gun.
Also, you can't vote on any posts before you registered, because they were from "before your time" here. In theory, you shouldn't be invested (emotionally or otherwise) in threads that existed before you joined the forum...unless you already had an account here.
It also gets lurkers off their butts to register -- because the sooner they register, the more posts in "that thread they're following avidly" they can vote on.
Any post on the entire forum a lurker is reading right now, he will never be able to vote on it. But every few minutes a new post is made to that thread, and if he had signed up 5 minutes previous, he would have been able to vote on it... better hurry up and register for an account!
-Matthew