Other than the number of riders on the grid, there's not a single difference in the groups you listed...

I watch as much racing as possible (some say I have an addiction), and every race, regardless of which racing group you follow, will have the same leaders waaaaay out front and winning...Some races will have 2, 3, maybe 4 fighting for the win, but not often...

GP is the top racers in the world, which means big egos trying to out-do each other, taking chances you don't see too much in the other classes...That's what draws me to watch...