I have a LHP (14 year old) and I concur that there are 2 main differences:
1. For every 5 or 6 RHP's you see, you see one LHP. The angle to the plate is very different and the extra split second it takes to adjust is usually all he needs to slide it past you.
2. Most LHP's I have seen do get more movement on their pitches than RHP's. To compound the scenario, all the movement is to the direct opposite of what the batter is programmed to expect as well.
My LHP has the second best velocity on my team, but his record is astronomically better than the kid that has the greatest velocity. Compare an 11-3-1 record to a 6-8 record - and there is almost a 10 mph dropoff in his velocity. When I pitch him in a closer mode behind the faster pitcher, he has proven to be virtually unhittable.
http://www.highviewheat.com/index.asp
http://www.kristensfastpitchworld.com/index.asp