Yeah, everyone got there before me. Cars is definitely their worst, I think it's because the characters were too unrelatable. I can't put my finger on it but Cars just lacked something. Maybe it just wasn't epic enough (as in scale not as in "OMG that's EPIC!"). Toy Story had the the whole world for the characters to overcome which (due to their size) seemed nigh-impossible, Finding Nemo had an entire ocean to deal with, Up had the South American jungle, Bugs Life had your back yard (but from their point of view, it was an entire continent).
I can't remember much of cars (maybe that's another reason it's not as great) but it didn't seem to be all that epic; even the plot was a little predictable and on a pretty small scale. All the others I mentioned went through a lot more emotional levels and had a hell of a lot more subtext, Cars was just (for lack of a better word) weaker than the others.
But that's just me.