As the newbie, I'll take the contrarian view.
The best, most active communities that I have seen are the ones that embrace and encourage all new members. They patiently answer the repeat questions and help the one-and-dones troubleshoot their problems. In doing so, those communities build up a priceless base of helpful threads and increase their chances of being noticed by potential new members from generalized web searches.
At the start, there is no simple filter to determine who will become the next enthusiastic participant and who is a one-and-done. Your instincts about a new member may be right, but you never know for sure. The more barriers and discouragement you impose on new members, the more likely your forum will devolve into an echo chamber for old timers who dominate it and drive others away, especially newbies.
I have been here four days and spent a lot of time reading many threads and learning a lot. If this website had followed your suggested policies, I would not have joined nor stayed long enough to learn whether there is a place for me here.
Welcome aboard and thanks for contributing to the discussion. Also, congrats on being the exception rather than the norm. I was a member for 6 months or more before making my first post so I've been there and done that myself. At the risk of sounding like a curmudgeon, I'd like to touch a bit deeper on a couple of your points with some follow-up thoughts of my own.
1. The best and most active communities are the ones that have members who participate in a variety of ongoing and sustainable topics on a regular basis. Spinning wheels on the same rehashed questions and drive-by members are not that.
2. Having to wait a few days to post is not the end of the world. Nothing we do here is of such a time sensitive nature that it can't wait a week to delve into despite how critical it might seem to someone in the moment.
3. This same waiting period would give a person the opportunity to search for an answer and therefore come up with a better or more complete question to pose. A better question = a better answer. And yes, it would weed out a good portion of the one and dones and classified droppers per se.
4. As for the database of threads for web searches etc, I would like to think that a few ongoing, detailed, and accurate threads on a topic would be more useful to a new or potential member than a whole bunch of similar threads with conflicting or incomplete responses on the same subject.
5. What you see as instinct, I see as a historical pattern.
6. In regards to the echo chamber of old timers, sadly, you are 100% correct. We absolutely do suffer from this as a group. I've seen quality thoughts or ideas shot down unceremoniously without cause because it doesn't fit into someone else's way of thinking on more than a few occasions. At times, we offend a new member in their very first post by being short or critical. Is it the question itself or the redundancy that leads to this? I sure don't know the answer.
Please don't read too far into anything I say. I don't mean to disparage at all and am actually thrilled for the engagement on the topic. I think this discussion is probably overdue and I would love to hear more people chime in one way or another. I'm even open to having my mind changed along the way.