In descending order of racingness:

F1 (Upcoming season is going to be a doozy)
GT1 Sports Cars (Le Mans, etc)
Touring Cars (the nearest thing we have to NASCAR; stock bodies, racing everything else, but on regular (i.e. non-oval) tracks)

It's interesting how CART drivers, for example, come to F1, usually score big in their first year due to the wow factor, then get their lunch eaten by real racing drivers. CF Montoya, Villeneuve.

I'd love to see a real driver (say, M. Schumacher) get behind the wheel in some of the US races. He'd toast 'em all.

Well, there's a few things.

1. Entirely custom cars. Knowing that the whole damn shooting match was built for 1 thing - go 200 miles as fast as possible.
2. Wet racing. A modern F1 car is a bitch to drive in the wet; it's what you get when you have >900BHP attached to ~600KG of car. A real wheat-from-chaff exercise.
3. Spa. There isn't another circuit on the planet to compare. Anywhere.
4. (Jingo alert) the fact that it's largely a sport driven by British technology and expertise; most of the teams have their factories in the UK. Even the BMW engine is designed and built in the UK, and Ferrari's wind tunnel was near Bristol for years. Ross Brawn, who's just presided over the most successful period in Ferrari's history, is British.
5. Ferrari. Love 'em or hate 'em, (I fall into the latter camp - Williams all the way, baby!) there's no racing team on the planet with the history or glamour that they have.