I simply prefer the noir/detective/relatively-grounded Batman, one who exists in a pseudo-realistic world, even if (and sometimes especially when) that world is ultra-stylized from time to time. But more than that, I like how simultaneously stoic, driven, and screwed up he his as a human being, and I love how his rogues gallery is made up of distorted reflections of the things he fears. The villains Batman faces are always so specific to HIS character, HIS psyche. In that sense, It's not just about overcoming them physically, but mentally as well. So when he faces/defeats one it feels like he's facing/defeating a dark, twisted reflection of himself, more so than any other comic book character I can think of. When you start tossing in aliens and villains who are synonymous with other superheroes, it just ends up feeling a little off and kind of waters him down and makes him feel out of place. Not to mention, powers-wise, he never quit fits when part of a team of *literal* superheroes either. I just think he's one of the few superheroes who belongs in his own world, if for no other reason than that world is tailor-made to serve as a funhouse mirror of himself. Gotham is Bruce Wayne's id, his ego, and his super-ego. Character and environment gel together in this awesome, twisted, but relatively closed-off world. Throw Superman or Wonder Woman or whoever else into the mix, or take Batman out of Gotham for an extended period of time, and it almost feels like you're literally messing with Bruce Wayne's identity, plight, etc. I'm not saying either of those things can't be done effectively, but I'd argue that he still needs to be the centerpiece in either scenario (as opposed to just a "teammate"), and Superman or whoever would need to serve as just another reflection, if that makes sense.
Anyway, if all of that makes me "closed-minded," so be it.