Mario Kart is more than jumps and short cuts. The cute characters and the "weapons" are good additions too. When I played MK, there were stuff happening everywhere on the track, contributing to the chaos and fun. It's also relatively short, so we could always go for another quick round; and another, and another. At least that how I remembered it. ^_^
Modnation could certainly try to reproduce that experience on PS3. If they are really good, then I supposed the game would be addictive without the modding.
I agree that kids like to fool around with the editor. My son likes to go into Create mode on LBP and float too. It's just that without the "mad" factor, the play-create-share concept may go stale after sometime.
I need to see the editor. If they are serious about user generated content, I feel that "deep" variety is important. The last time I saw a modnation demo, it gave me the impression that it's mostly about decorating the landscape and doing track layout. e.g., I am not sure if I can drive over the water I put in. I don't need *a lot* of variety, but a few deep building blocks would be good.
If the editor is shallow, the user tracks may play similarly. People will just go back to the built-in tracks.