and maybe additionally joining a new proposal should either cost some rep or be limited (per week or in total)
Agreed on voting and posting questions only on followed proposals.
But I'm totally against limiting how many of them you can follow.
When you create a proposal, you automatically follow it. Do you mean that you should have to wait until someone else follows it prior to creating sample questions?
Those first few questions help you to clarify what you had in mind (as far as what should be on / off topic) when you created the proposal.
I'd also be against a limitation of how many proposals I could follow. Of the (I think 10) I'm following now, I'm committed to only two. As it stands, I don't think I'll be committing to anything else that I follow, but I like to keep up with the proposals anyway .. I may change my mind.
In short, just following something isn't going to get it into beta. I think the fact that we can only commit to a few proposals will naturally keep 'questionable' stuff out of beta.
Now - reading between the lines -
It's been made abundantly clear that no matter the intentions, creating a 'fun place' is not in the scope of Stack Exchange 2.0. Likewise with a deliberately nonsensical site. If I start a proposal today on .. let's say ... "How to be a pimp" - even if it got 200 followers, you can be sure that it would not live past the first 24 hours. Keep in mind that early on, nobody but diamond moderators could vote to close a proposal. That has since changed.
I think anyone should be able to post a example question, but I can see a good case for only letting people that follower a proposal vote on its questions.
By following I am saying I will take part in the site often, the fact I will only make use of a site 1 or 2 times a year does not stop me posting good questions, however it means I should not follow it (and imply I will commit), or help define it.