Perhaps my own experience is atypical but I'll elaborate anyway.
SO had been out of beta for a 4-5 months before I really started answering questions. I had dipped my toe in the water a couple of times prior with varying results: one particularly successful answer, several unsuccessful answers.
Granted, now is not a few months after launch. Questions tend to be more esoteric, there are more question snipers/campers and so on. But IMHO the same principles still apply: be quick and be concise. You can expand a concise answer with a longer answer but you can't usually ignore the concise answer.
Now some consider that a bad thing. Me? I think it trains people how to give good answers and not just for SO but in their work life in general: front load the information people ask for and then expand on it.
As for getting the initial rep: getting rep to vote to close and so on takes awhile but do you need that? The more annoying levels are when you can't comment or upvote. That you can get fairly quickly just by asking 1-2 questions if not answering them.
My own experience is that newbies who ask good questions or give good answers I (and others) will upvote them simply to get them past these points (to about 50 rep).
As for the barrier to entry and community aspects, there are issues here. In fact, I raised this very issue 1.5 years ago [1] and I still have mixed feelings about it.
Ultimately though, an abundance of answerers I don't consider to be a problem. With attrition, others will replace them. I'd only worry if no one is answering.
I don't see the value of SO is being in the community per se. Not in the same sense that the value in Quora (IMHO) is entirely in the community (which presents real long-term risks).
What SO did well, which obviously affected the community that grew around it (and Joel has spoken about this), is that it made bad behaviour hard, something people would actually complain about. Example: a common complaint is that SO makes discussion hard. Well, that's actually kinda the point. Discussions are 99% noise in the context of Q&A. SO allows good answers to float to the top with the voting mechanism.
Remember: rep isn't an end, it's a means to an end. It's also a way of solving problems that would otherwise require human intervention. For example, the minimum rep for editing answers. This isn't a de facto claim that a certain rep threshold means you know what you're talking about. It's simply a way of expressing the site's trust in you, as a contributor, rather than attesting to the accuracy of your technical knowledge.
Better that than Wikipedia-style revert wars that require constant mod attention.
SO is an excellent system for askers and answerers of programming questions. I'm simply unconvinced that this models translates well to other areas (as per the StackExchange model for other sites).
[1] http://meta.stackoverflow.com/questions/1483/are-power-users...