Wow. Looks like I missed quite a debate on graphics/attention to detail last night!
Personally, I'd agree that for the most part, the PS2 games are more exciting to look at.
ToD was pretty good in that respect; but I think ACiT fell very far short on that; Axiom City was nowhere near as good-looking as Metropolis, Megapolis, Notak, Igliak or Stratus City…
As for the "nature levels", some of them are okay. I quite like the first part of Coballia (the part that you can't return to until the next playthrough!), other than that, most of the nature-type leves in the PS3 games aren't as good (with the obvious exception of everything throughout A4O, which was GORGEOUS), but even the PS2 games had nicer nature-levels IMO: levels like Eudora, Novalis or Hoven were quite nice.
And then there were certain themes that were never really brought back up; despite the PS3's rendering abilities, we never got anything that resembled the types of environments we got on locations like Batalia or Gemlik base.
Sure, Gemlik Base doesn't have a lot going on like some of the cityscapes, but it's nevertheless quite a spectacle at times; the only time we get anything like that in the PS3 games was the space-combat levels in ToD, but they go by so fast that you don't really have time to enjoy them, whereas, Gemlik base is fairly tranquil, allowing time to stop and look at the gorgeous spacey backdrop.
Then again, R&C3 and Gladiator/Deadlocked weren't as interesting as the first two games, so maybe we shouldn't be comparing PS2 versus PS3 so much as each individual game by it's own merits? (e.g. notice, almost all the examples I used from the PS2 games were from the first game?)