Dedicated scammers or trolls will still be able to spam out the required 25 posts. Newbies with something of interest to say will mainly be frustrated and may abandon the forums altogether. I don't see this as being a smart move.
edit: lol 50
+1
I can offer myself as an example. I have been reading about bitcoins for the last couple of days and have been giving some thought to buying some. I also wanted to offer some feedback to a forum member who has started a bitcoin giveaway site designed to help people like me 'get on the ladder' so to speak.
This policy absolutely drives me away as a matter of principle. This forum appears to be THE source for BTC related information and you are shutting people like me out who may occasionally want to contribute or offer feedback. I have no intention of making 50 posts just to 'earn' the right to play with the big boys.
Do you want bitcoins to gain wider acceptance or do you want it to be an exclusive club that can be entered into only by those 'in the know'? I get the impression of the latter at the moment.