About that general barrier to commuting -- I've only been doing this for about 6 months (I used to commute, but in much smaller cities - Oakland, Providence - and that was over a decade ago!) so it's really fresh in my mind that commuter riding in traffic demands a whole set of skills and habits that other riding might not. I guess roadies have to deal with cars, too, but that seems different from city riding (I've never been a roadie so I don't know but that's my guess). Before I started this year, I was scared of NYC traffic. About five years ago I was telling someone about a cycling friend who had nearly gotten killed by a bus -- his bike skid under the wheels and was wrecked but he wasn't too hurt -- and I distinctly remember saying, "you have to be insane to ride a bike in this city".
I'm not saying I'm _not_ insane, but after I started riding, I realized that I essentially just needed a training period in order to learn how to ride securely and confidently in rush-hour traffic. It helped to read other cyclists' comments on websites and boards like this, but mostly I learned just by doing it, day after day. Over time I figured out the _real_ rules of the road, and how one can get around this city safely, calmly, and without too much hassle.
I think that people who ride bikes in other situations might have a bad experience the first few times they try to ride in heavy traffic and decide that it feels too dangerous and scary, so forget it. I don't blame them, since I was recently like that, and that's also where the on-average-more-cautious-female thing kicks in -- "Risk my neck for that? No,thanks." But they may not realize that it'll get so much better once they get used to it and they go through that necessary period of learning. And of course, car drivers who don't ride only see how vulnerable cyclists look in traffic, exposed to the air with no metal around them. At least I assume that's what drivers think -- I haven't had a car in 12 years, and, speaking of geographical differences, I don't know many New Yorkers who drive.
Okay, now I'm going to go knock on wood so I don't end up under a cabbie's front tire tomorrow morning.