Winnowed from a thread on the Road bike forum:
1. It's against the law
2. Riding against traffic reduces the reaction time of cyclist & driver since you're approaching each other instead of going in the same direction.
3. Potential impact is greater: Bike going 20 mph and car going 40 mph
collision riding with traffic = 20 mph impact
collision riding against traffic = 60 mph impact
4. Coming over the crest of a hill, if riding against traffic you'll come head on to an oncoming car, whereas when riding with traffic, the upcoming car on your path of direction will see you going up the hill.
5. Drivers making RIGHT turns will only look to their LEFT. Since they have to merge with that traffic and are not expecting vehicles to be coming head on from their right side
6. Drivers pulling out and making left turns will only look to their left, thus pulling out in front of you.
7. Riding on the right, a car can slow behind you and wait until it's safe to pass. Riding on the left, you're coming right at the cars, leaving them the choice of a head-on with oncoming traffic or a head-on with you.
8. Primary tenets of safe riding are to be visible and predictable. Riding on the left puts you where drivers aren't looking for you, and you're demonstrating a complete ignorance of traffic laws; so you get a FAIL on both counts.
9. It's probably much more likely you'll get doored driving against traffic as well. Most people are looking in their sideview mirror or behind them for a passing car before opening the door, since they aren't expecting anything coming from the front.
10. The reason you ride with traffic is because when on a bicycle, you ARE traffic. you are subject to most all of the other laws that govern vehicles, so you must be in proper position to obey the laws AND be protected by them. Riding on the left, all traffic signs and lights for your direction are on the OTHER side of the road.