...I'm curious, what muscles fatigue for everyone else?
My quads are the first thing too. I've found doing a lot of intervals on the bike and that bosu squat exercise to help.
Another thing that hasn't been mentioned is plyometrics. They are a lot of fun and great for snowboarding. I know T. Rice has mentioned he does them a lot. If you are a subscriber to SnowboardAddiction, they have a decent workout that combines strength, balance, plyometrics, and flexibility.