Well, because of this 20 post limit, a lot of useless posts are generated...
There's not a 20 post limit. You need one post and 4 hours wait.
I've read that you need 5 posts earlier, now I see you are saying it is only one post?
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A quick read of the rules sorted me out.
https://bitcointalk.org/index.php?topic=177133.0Newbie RestrictionsYou can only post in the Newbie and Local sub-forums till you have made one post and waited 4 hours. This post must be substantial: at least several sentences. The other two restrictions are you need 1 post to be able to send a PM and 10 posts to put a link in your sig. You can only post once every 360 seconds to start and there are a few other things that trigger this timer such as logging in and searching.
Seems pretty straightforward and I agree with the reasoning.
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