That said, I think both of them went off the rails eventually, despite having many fine products and an amazing amount of potential.
The initial boxed set for Spelljammer did a really good job of explaining that adventurers that ended up in Wildspace and beyond could be from any existing D&D world, or the DM's home campaign, and "boom" they find or buy a helm, fly off into space, and you get to use the Spelljammer material.
The emphasis in the boxed set seemed to be that some planes were harder to get to, and some planes didn't have a direct route to them. It was easiest to get to the Forgotten Realms, but leaving Krynn seemed to be more or less a one way trip.
"Civilized" Wildspace was the common ground of most campaign settings writ large. Giant space dragons, mind flayer and beholder ships, dwarven asteroid mining facilities, graceful elven armadas. Then the boxed set hinted at the weird and wondrous stuff. The stuff that could be unique to this campaign setting.
It mentions crystal spheres where planets hang in trees, or ride on the back of a giant turtle. Where all life might have died out, or no recognizable fantasy races are around. In short, there was an endless universe of truly fantastical fantasy settings that people could fly their magic boat towards, all while saving these pristine oddities from traditional D&D bad guys that had some new quirks for space.
The problem is, some of that wonder was forgotten when the sourcebooks and adventures hit. Instead of saying that maybe one or two really "in the know" people in a setting know about Spelljammers and Wildspace and all of that, suddenly, all of the major settings have Spelljamming ports, and procedures for how to deal with them, and really hollow sounding excuses for why countries that are now said to own Spelljammers aren't using them for war or exploration.
Adventures started to forget that the boxed set explicitly mentioned that some crystal spheres couldn't be reached from other spheres, giving the DM an excuse to not let their PCs get back home for a long time, and just assumed you could go anywhere when ever you wanted to go, from where ever you wanted to leave.
Spelljammer really should have been more explicit that the Arcane would show up and sell to somebody once in a generation, and had no interest in selling to planet bound nations. Play up that they kept their eye on adventurers to find ones that wouldn't just turn a ship over to their home kingdom, but would actually head for space and explore. Heck, it might have even led to some more interesting plot hooks to see what the Arcane would do when an adventurer did try to turn over a Spelljammer to a land based authority, perhaps calling in Giff mercenary squads to deal with them or at least destroy the Spelljammer helm.
There should have been more adventures like Crystal Spheres, supplements like Practical Planetology, and the Legend of the Spelljammer boxed set. Supplements and adventures that game the setting more unique things to call it's own, and less trying to fit square pegs into existing campaign setting holes.
Finally, in my humble opinion, there really should have been a slightly different explanation for how to transition to the Phlogiston. While having a unique "everything fits inside a crystal sphere" cosmology would have been cool for some worlds, Spelljammer committed one of the same sins that Planescape would repeat, creating "meta" rules that forces individual D&D settings to rethink how things work. If the setting assumes that, yes, there is magic, but the planet is still a planet orbiting a sun, and that starts are still giant balls of plasma light years away, suddenly that can't be true if it's a standard D&D world.
I started to warm up to the idea that it might have worked better to have stable and/or unstable "Phlogiston storms" that allowed access, and also allowed for an excuse for why adventurers might be able to leave a world and have a really hard time finding their way back to it, especially of one of those storms is unstable and not charted anywhere.
Planescape managed to hold together it's unique qualities a bit better in the long run than Spelljammer did. Part of this is probably because it was a bit easier to have a pure Planescape campaign. Between the gate towns and Sigil itself, there were plenty of places for a 1st level adventure to get their start and be a "native" to the setting.
Planescape had the allure of the Mos Eisley cantina, in a fantasy setting. You could talk to a Pit Fiend on neutral ground and not worry about him melting your face off, and you could have drow, half-demons, and maybe a dragon hanging out across the room. There was a great tension about having great power and hostility held in check by the factions and the Lady of Pain herself.
Having other planes of reality that were based on belief systems meant you had a lot of potential for high concept adventures and weird locations. It even felt a bit less "theme breaking" to visit individual "standard" worlds, because if you became embroiled in the factions or worked for some powerful extra planar being, you had a built in reason to favor your "native" community rather than just slum around in the individual campaign setting.
The temptation for "meta-campaign" imposition seemed to be lessened in Planescape as well. Sure, Krynn didn't have the same references to the planes that other worlds did, but Greyhawk and the Realms both "knew" there was a Seven Heavens, a Nine Hells, the Abyss, etc.
And yet, an imposing "meta" campaign set of rules still started to emerge. Krynn couldn't be connected to a different set of outer planes than the other D&D settings, the gods there just enjoyed renaming their planes and hiding the truth from their followers. Have a belief system that says you get reincarnated or that you live happily ever after? Well, we know it's not true, because Planescape establishes that every soul forgets who they are and starts to fade away after a while. Forget individual god's teachings, different pantheons, or individual worlds. If you are a D&D world, this is how souls and the afterlife work across the board.
Also, the longer the line wore on, the more "truths" about the panes became apparent. In theory, it makes perfect sense if the Nine Hells has it's real history, and two different origins based on what people in Greyhawk or the Realms have heard about Hell. But the problem is, if your campaign setting had an origin for the gods or a plane or something important like that, not only was it likely to be shaded based on the campaign, it was likely to have a definitive answer that meant your campaign setting was wrong about it, and unfortunately, instead of just portraying people as not knowing the absolute truths of the outer planes, the very secrets of the gods, campaign setting material started being rewritten in order to accommodate the meta-truth of Planescape.
Revealing too many truths about the nature of the universe wouldn't have been a problem, if those truths were then translated to to truths in individual campaign settings. What's even worse, that caused the over-correction that was the 3rd edition cosmology of the Realms, which actually made things worse rather than liberating the setting from meta-campaign concerns.
To me, the Malhavoc Press d20 supplement Beyond Countless Doorways underscored something that might have been a valid point to realize during the Planescape days. While there were always demi-planes and alternate prime materials you could wander onto, in many ways it felt as if there was a set, finite number of Outer Planes. Now, places like the Abyss had so many layers this didn't always feel limiting, but if characters could find conceptual planes that weren't just based on alignment but other philosophies, the dangers of imposing a Truth on existing campaign worlds seems as if it would dwindle.
What if there were fragmentary planes based on unrequited desires, shame, or exuberance? How would a plane operate if instead of a more complex set of philosophical guidelines like an alignment, the whole plane was geared around fostering one philosophical, psychological, or emotional concept? And would these planes have a natural connection to multiple planes of existence, perhaps creating a bridge between Upper and Lower planes if their concepts were devoid of some kind of exclusivity to good or evil?
Planescape got high concept, but at times, it felt like high concept within a specific D&D shaped box.
Who Did it Right?
I'd argue that the best "meta-campaign" setting that TSR put out back in the day was probably Ravenloft, because it largely managed to stay true to it's original vision statement. Maybe that's because it was like the roach motel demi-plane (PCs went in, but a lot of times they didn't come out). Because you could use the rules for Spelljammer and Planescape to explore existing settings, official supplements did this, and exploited the fact that they were "meta-settings," sometimes to the detriment of fully developing what was unique about the setting itself, or to the point of imposing something grand across D&D as a whole.
Ravenloft grabbed PCs from all over TSR's settings, told them they were trapped, and then let them bring the uniqueness that their setting might lend to a character, but isolated them in a world with it's own rules. Even chunks of other campaign settings that ended up in Ravenloft had only some of the flavor of their original campaign setting, mixed with the rules and the tone of the Demiplane of Dread.
At least to my jaded gaming eyes, Ravenloft did a better job of maintaining it's own feel and mission statement while borrowing elements from other campaign settings, and not imposing any new rules on any of the settings that it touched upon.
Disclaimers for Everyone!
I am an old gamer, and I didn't reread all of the material I muse about, so it is entirely possible that my recollections of things have faded or become muddied over time. If this is the case, allow me to apologize for any hazy recollections that may have found their way into the blog.
Also, all three of the above settings has some amazing supplements, some meh supplements, and some bad ones as well. Despite any criticism I may have levied against it for a particular reason, I also think Planescape was probably the most consistently good product line of all of these mentioned, and it was that quality that likely earned a lot of it's "primacy" when it came to imposing the truths of the multiverse upon other D&D worlds.