I ride an '03 CRF450, and use an 9 oz. Trailtech flywheel weight along with a rear sprocket change to 51 teeth. I ride a lot of technical trails and have had no problems with traction or stalling. For example, at the WFO enduro this year, which was fairly muddy, I was able to literally stop on a muddy uphill behind a downed rider, and find traction to climb the hill even out of a large rut. I am coming off a 300 EXC and a 400SX, and feel the motor on the CRF works as well, if not better, in those condtions. My opinion, of course, and I am sure others will provide disenting views.
I have had zero problems starting this bike. I came off a KTM 400SX that was bumped to 450c's with a Thumper Racing kit. It sometimes gave me real fits starting, but the Honda kicks easily, and is usually a one kick starter. It even starts within one/three kick after it has been dropped. The only time it takes more kicks is when I give it a half hearted boot. After that it has taken a few kick to get going. I'm thinking I must push some extra fuel into the motor by the weak kick, and then it takes a few kicks to clear it out. I race hare scrambles and enduros, and it has always started on the first kick in H.S. In enduros I usually stall the engine once somewhere. This is a lot better than I used to do on the 300EXC's I rode in the past.
You can see, I'm really impressed with this bike. Also, yesterday I was out riding with some "A" District 36 riders at a local riding area. There is one section about three miles long that features small trees about 28" apart, along with roots, ruts and log crossings. My Honda did not overheat, and held off a group of very good riders on KTM450's, Kawasaki 250F's, and Husaberg's.