... also, does the various floor tile design mean anything, or is it just eyecandy?

They're just different difficulties, there's nothing special about them. Aside from different score multipliers and some visual and audio effects.

The higher difficulties seem to be easier to me.
That is probably because I tend to get farther from the better loot.

It's probably due to the fact that you're switching over higher difficulties when you're more comfortable with the game and unlocked a few new gods / upgrades. I'm myself just stopped playing anything but Chaos. Half of the reason is music, I really like how different tracks are in Chaos.

I'm still new, but I figured out there are two floor tile sprites that mean a corpse is there. One with a skull, and another one with some unidentifiable garbage. This can help you keep track of how many corpses are there, since some monsters gain +1 ATK per corpse present on the current floor...

I'm kinda starting to get a similar impression. Higher difficulties seem to be "meatier" in regards to loot and progress. Harder pass/fail divide.

...not that I've actually made it to the lowest level on any difficulty yet. >_>