As someone who's favorite 2D Mario is 3, and has become complacent with the fact that the majority will always prefer World over it, I can't help but thank you, OP. You've pretty much laid out my thoughts about World verbatim. Granted, not all of them, but it's almost cathartic to see a World fan realize all these little things about it.
One thing that bothers me is how it doesn't really push its hardware much, especially compared to Mario 3. Yeah, it expands on 3's mechanics here and there, but there's nothing that really showcases the SNES as anything more than the Genesis. I grew up with the SNES version of Mario 3, and there was something about going back to the NES version, and realizing "wow, mechanically, this is lightyears ahead of 1, and running on the same hardware".
Also, the visuals aren't awful, but feel wonky, and even lacking compared to games like Sonic 1. It seems like the devs were baffled by the palette bump, and never managed to develop a persistent art style for the game. It almost feels a little inconsistent between sprites, and the shading is overly simplistic, leading a to a rather flat look. This one's more subjective than anything else, but its always bugged me.
Despite all this, I still love World. I've gone back to it here and there in recent years, and have grown to appreciate it a lot more than I used to. It's a pretty great entry point into the series, mostly due in part to the Cape powerup. It'll never dethrone 3 as my favorite 2D installment, though.