I believe that's a personal call + you gotta look at your users.
For example. If you have a lot of random hits from people searching for content, and just happen to run by your site, and want to leave a comment or post about something, the registration would be a deterant, because let's be honest, no one wants to have to sit through a registration if they never intend on coming back again.
But then again, if you want them to come back, it could be a good thing.
My whole take on it is that no one should be allowed to post anything without having to wait atleast 2 or so mins. Most constructive thoughts don't come out of your fingers in less than 60 seconds if you're posting information on a forum/board type environment.
But it could be possible to limit a non registered user to 2 posts a day or so. Just keep a db table to store their IP or maybe just leave them with a cookie, though they could just clear the cookies. Both ways have draw backs, along with upsides.