As Monica wrote, new users absolutely must be able to post questions the minute they create an account, because most people will not create an account until they have something to post. New users may be subject to a rate limit which progressively goes up as their “asker worthiness” increases (as is the case on SE) and may be subject to more intense scrutiny while their asker worthiness is low (as is the case on SE).
Spam (including scammers) is largely a solved problem. SmokeDetector works.
What isn’t a solved problem is voting — let’s discuss it in its own thread. If you can just create an account and start voting, this opens the door to sockpuppetry, and sockpuppetry is far from a solved problem on SE. Even if we don’t have a prominently displayed reputation figure, so there’s less incentive for people to vote for themselves to inflate their reputation, there’s still plenty of other incentives. I don’t have figures and I don’t think anyone can tell for sure because not all puppet masters are detected and not all puppet masters have an obvious goal, but I suspect that the largest number of puppet masters on SE are people who want to get around the question blocks. We’ll have to face this problem no matter how much our site differs from SE as long as we have some concept of question quality, which I think is consensual.