In particular, I really, really dislike how the Lady of Pain devalues the gods. I'm not opposed to areas that gods can't enter (see Death's Reach), but the reason they can't enter there is far more convincing than the fiat that enables Sigil.
I personally am happy to see that the gods are not all-powerful in a cosmological mash-up. There are Forces Beyond Gods. It is infinity turned up to 11.
I think it's kind of essential for a cosmological mash-up to have gods that are not all-powerful, especially if Philosophy is one of the defining pillars of the setting, since you want people to be able to doubt and question them, rebel against them, pronounce them dead, compare and contrast them, ignore and defy them, or believe in them for reasons beyond "He is big and shoots lightning and can kill things and lives in a paradise far away."
That's my take, anyway.
Then too, Planescape is part of the extreme fragmentation of D&D, so I've always been opposed to it on that principle. (I also am not fond of DiTerlizzi's artwork, and I despise the cant).
Eh...tastes vary.

I don't know what you mean by "extreme fragmentation of D&D" though?
One of the best times I had recently was the PCs meeting the Raven Queen, and them being speechless as a result. This was a god - their god - and she held the power of life and death over them. That's what I want from the gods: these are the creators of humanity and demihumanity, and they should be loved and occasionally feared.
That's great for a setting where most of the adventures take place in the Mortal World, but Planescape rejects that idea. Immortals are fairly common beings, everyone has their own view, and the gods, while very powerful, are ultimately nothing special. Part of that bag is the use of philosophies as a core element -- you want to include characters who are inspired by fantastical takes on ideas of right and wrong and life and death beyond the simple animal awe. You want to have characters who can spout Buddhism and Nietzsche, and that requires a different take on the nature of the gods. In PS, the awe is not that there are powerful entities, but rather that there are things
beyond the most powerful entities.
This is not to say that the gods are omnipotent and invulnerable - by no means - but can you really imagine the Zeus of Greek mythology or the Odin of Norse mythology being unable to affect the Lady of Pain? The influence of Greek and Norse mythology is extremely visible on the 4E mythology, and it'd be nice to see the gods in a similar position of power.
Why do gods
have to be that powerful? Why is it a prerequisite for your fantasy funtimes? Why can't the gods -- in certain settings -- be petty, vulnerable, and small-minded? Why can't clerics believe in their deity not because he's the biggest, strongest thing around, but because he represents something the cleric feels is sacred on a level that suffuses the entire multiverse, rather than just the little island where that god is worshiped?
Returning to Sigil, one effect of the Lady of Pain's existence is that suddenly atheism - in an otherwise theistic cosmos - becomes an option. And, thus, you get the factions. Completely irrelevant outside of Sigil, because the morality and ethics promulgated by the gods actually apply elsewhere. It sort of works in the confused state of mythological affairs that was 2E, where there seems to be 1001 gods or more, but with the tighter set of mythology in 4E, even if the gods don't work as tightly together as a pantheon as the rest of mythology implies they should, the factions stick out like a sore thumb. Thankfully they're mostly not there in 4E.
I get that if you don't like weak gods, that not liking atheistic or agnostic factions would follow, but I don't get why you can't have weak gods? Maybe it's just a "personal taste" thing.
The idea of meeting places - civilisation - in the Astral Sea is an interesting one. Personally, I'm very fond of the City of Brass (dating back to the cover of the old Dungeon Masters Guide), but with Sigil, I really need something more than "it exists because of the Lady of Pain". Who uses it? Why do they use it? What is the civilisation of the outer planar creatures that requires such a meeting place to exist?
Trade and travel hub of the multiverse, a "safe haven" for all walks of life that links to every other reality that could ever be, as easy as walking through a door.
"An angel and a devil sit down at a bar" sounds like a good start to a story, but why would that meeting ever take place? Is it a forced contrivance to tell a story, or does it make sense within the mythology you have? In 4E, it looks forced to me.
Philosophies is the reason. The angel and the devil have more in common in PS than they might in other settings, since they might, for instance, both believe that power should be dictated through an authority. They sit down at a bar to discuss why they each love their leader so much, and find their ideas matching up snugly, much to their disturbance.
The civilisation of the Efreet gives rise to the City of Brass, and the trade opportunities there for high-level characters, and thus I can justify it. I don't have such a justification for the City of Doors.
City of Brass isn't connected to Everywhere, it's just big on the elemental scene.
----
I'm not exactly trying to
convince you to like it. Y'know, like what you like, I don't care. But maybe we can drill down to a fundamental place. You don't seem to like it mostly because you don't like trivializing the Gods. What's wrong with trivializing the Gods, for you?