Fame makes it a lot easier, but you aren't doomed to failure if you aren't famous. If you have the dollars to do good marketing then I think it would be easy to make it work.
I agree that start-up is the problem, but sometimes users who just wander in can make a question-and-answer site very successful, even if the operator is obscure. What's crucial for getting lucky this way is good usability of the site and a crew of volunteer moderators as the site develops. My best example of success in a Q and A site is College Confidential.
It isn't perfect, but it is chock full of information about the college admission process, and its online atmosphere of politeness and regard for other users' questions readily differentiates it from most other competitors running sites on the same subject. The site operator is pretty good at getting press notices and other free publicity, and quite good at search engine optimization, but mostly he's good at building a helpful and friendly online community.
I don't think it would be too hard to attract a 'name' if you got a little bit of headway. If you chose, say, mountain biking, it wouldn't be that hard to get some endorsement from whomever the mountain biking celebrity du-jour is. In niche fields like this the top guys are pretty accessible.
Mountain biking has a solid fan base, worldwide, and the guys that are into it aren't afraid of spending several thousand on a bike, plus all the bits and pieces that go with it. They're also generally young, single and male, which dovetails nicely with an internet demographic. After all, you can't go out at night. That means there's ad dollar to be made. There's also a vast amount of technical know-how that goes into the sport, both from a how-to setup your bike, how-to do particular techniques, and where to go. A lot of this is currently buried in forums with crappy interfaces and broken searches.
A niche area like this isn't going to get you bought for 10 million, but a couple of talented guys could make a nice living doing something they love, which isn't to be sneezed at.
There are so many other niches that spring to mind crying out for a better problem-solving model than forum postings, which is really what SO is. It's true that Yahoo answers could enter any of these markets, but with branding and endorsements, a dedicated site would outstrip Yahoo answers without much trouble.
I expect that if you're creative, dedicated, and put in the time to tackle the "long, hard slog" you'll have something of considerable value by the end of it. In fact, if you can get the momentum necessary, you'll have precluded others from even entering the field.
But you're definitely corect that the right persona and pre-existing followings basically super-jumpstart these types of sites.
He had his own segment on The Screen Savers, a fairly popular geek/tech show, for some time. That's where he plugged Digg. So yeah, he was pretty well known.
What you need are credentials - doesn't necessarily need to be fame. Admittedly -- It's probably much harder, but you could demonstrate credentials through some other means.
HN works because PG is here. Reddit worked because PG was there to help it start.
Digg works because Kevin Rose is there.
StackOverflow works because Joel and Jeff are there.
Unless you've got a large audience to toss at the project on day one, expect a long, hard slog.