Just a quick thought for newbie probation is to allow a new user to post 1 time per day in any thread, or in a certain section of threads. If that "newbie" is quoted within the thread they are allowed to continue the conversation with another post, because they obviously have posted something interesting.

After a certain amount of time, or quoted posts, it shows that they have something to contribute to the forum and can become a true member.

I'm generally a rare poster, so this 5 post limit is killer for me because I would much rather post something of quality but I find myself randomly running to posts and posting anything I can think of. Just my 2 cents, but 5 posts probably teaches more bad habits than anything.