MCO was great, and yes, like any MMO, it did have bugs. The difference became EA. EA refused to fund the game or its developers after a point. The thing is also advertising. You arent going to bring in any new people without advertising. And there simply wasnt any for the game. It spread by word of mouth. The gameplay mechanics were really amazing, with a crazy amount of engine customization, especially for an online game, which i dont think TD:U will be able to achieve. The auction system worked great (most of the time), and there really were 1 of a kind cars.
Test Drive: U isnt a MMO in the sense of MCO though, since i dont believe its a truely persistant community (One of the features cut from MCO was driving around before races and stuff), but im sure it will find its niche. As long as it gets decent reviews, ill probably try it out.