I don't think post restrictions would be a good idea. In order to make my point, I will attempt to make some broad strokes as to sets of users.
- Lurkers -- Never post, never will.
- One post wonders -- stop by, make a post, and then leave.
- Learners -- Like (2), but might have been willing to stick around. They are repulsed by the giant neon "No"'s
- Rare poster -- Tend toward few posts, longer posts. Spent enough time as (1) to read the posts and learn to avoid the bad topics.
- Mainstream -- Many, many posts. Some incredibly short, others longer and more detailed.
There are a number of subdivisions to (5), but I'll just lump them.
If one would make a post restriction,
- Lurkers: No change
- One post wonders: Either they become lurkers, leave, or progress more slowly to mainstream. Some might indeed be of the "lightning rod" crowd, but some might also be "Charcoal" or "Compressed Solar"
- Learners: Two forces. On the one hand, their first post may never occur, so they go lurker or disappear. On the other, you have the progression towards (4) and (5).
- Rare poster -- Would be forced to post more often before they could bring up their own topics.
- Mainstream: No change.
I think you would avoid the "Lightning Rods" but may also lose a few ideas. Some would stop by to make what would indeed be a good idea, but would never stick around to make enough posts.
By the law of unintended consequences, it is my thought that there would be an increase in two types of posts.
- "Me too", and "Plus one" posts. Posts for the pure purposes of increasing one's count.
- "That's a good idea, but this would be better" (ie, threadjacks). May require even more administration than the current. Why bother making 20 posts when you can just post in someone else's first thread?
On my own, I would advocate a change in culture. Instead of "Welcome to the forums, hold this nuke, use the search next time", how about replying with the links to the "Denied topics" thread, or at the very least a good search term to use. Prove that the search function works. It could even just be a form-letter thing you could cut-paste. I regret I cannot name names, but I have noticed this trend occasionally, and applaud it. I mean, when an admittedly new poster (may have been youth or a language barrier) is reduced to angry tears (yep, it happened), that can't be a good thing.
*shrug*
If the restriction were to be put into place, it should be lower than 20. 10, or maybe even 5, which may serve to reduce the above unintended consequences.
EDIT: A few edits to restate things in a less confrontation way, and to correct mistakes.