This could be done.
Restricting the use of an Avatar would be easiest.
Restricting the use of smileys would be tricky. But I think it could be done.
What you would have to do is turn off UBB code for users who didn't have a high enough post count.
You could probably even not display the UBBCode boxes (below where you make a post) for users without a high enough post count.
If you are interested in this, I'll write it up. Will probably be Monday or so before it makes it to the top of the list.