- Multiplayer support
- Skinning
- Easy track editing
- Decent physics
- Modability (something like Unreal Tournament.. Easy to activate and deactivate, would give options to add say Turbo and small stuff like that.)
... And that's it. Gimme a game like that and I'm in heaven.

Doesn't have to be realistic in the sense that there are Real cars on Real tracks. As long as it's believable, fast and good for online play I'm satisfied. Something like Powerslide, only more modifiable and open ended would be brilliant.
What's yours like?