The biggest difference between the two Burton Batman movies is said pretty succinctly in the "making of" commentaries. "Batman" was a Batman movie directed by Tim Burton, where he was reigned in by the studios mythology wise, but to get him back, Returns was a Burton film where he was allowed to do whatever he wanted with the bat characters in it, no adhering to mythology.
Laugh- And didn't that bite the studios in the ass?
Returns IS Darker just because at the start of the movie Penguin is already a child murderer then he wants to later slaughter all of Gothams first born. It's certainly more adult just through it's dialogue and innuendo. And yeah, then it gets kind of silly with Penguins goons kidnapping all the kids in a duck train (laugh). Nothings perfect man!
Batman Returns just has this whole vibe that pulls me into that Gotham world and grips me. There is just a whole romance to the way Burton created that world. (And I like the Batman costume better than the one than from '89.).
Returns is definitely a movie where I think people either love it or hate it.