I commute to work every day on my bike, and I live in Houston, which is probably about as car-friendly a city as you can get.
Most of the cyclists I see are pretty well-behaved, stopping at lights and behaving nicely.
The drivers, though, have no idea what to do with this strange two-wheeled human-vehicle centaurish thing which randomly appears on the road with them.
They'll swerve out into oncoming lanes of traffic if I'm by the side of the road. They'll pull to a near crawl and form a column behind me. They'll speed up and buzz my bike going 40 mph. They'll honk at me as they come up behind me, pass me, and drive past. One nice lady tried to run me down in her SUV because I had the gall to pass on her right. And if there're two (two!) bikes sharing a lane? They'll completely lose their minds at the sight of such a miracle.
Let's use hand signals, and show which way we're going--haha, just kidding, no drivers in the city seem to understand the difference between "I'm turning left", "I'm turning right", "I'm about to brake", and "being on a bike is so awesome I'm just going to flail my left arm around".
Also, the roads here are just barely good enough for cars--on bikes, you'll merrily eat a pothole and go flying, or bend a rim, or whatever. The sidewalks are even worse, otherwise we'd be on those.
And then there are the pedestrians! If drivers are clueless, people that are ambling around or jogging are malevolently stupid. Call out your passing side ahead of time, and they stop and crane their head around trying to see if God himself was talking to them--instead of, you know, moving out of the way in the indicated direction. Then there are the uppity joggers who will get in your face for sharing a dirt path ~4m wide.
The biking culture is getting better here, but it can really grate on one's nerves.