Consider this situation:
User signs up and spends 30 minutes writing up a post.
User hits submit.
"You need five posts before being able to use any of this board's vB code features. Including smilies and linking to URLs."
User presses back button, only to realise in horror that the post is gone.
Okay, I've learnt my lesson to always write posts in Notepad now, so it doesn't affect me. Maybe the forum administration doesn't care too much about newbies or whatnot, but anyway, just thought I'd point it out (if it hasn't been already). Perhaps inline the error so that the textbox still shows up with the content, or don't cause it to vanish. Alternatively, just don't parse bbcodes in the post made?