The primary reason that I love Spelljammer so much is that it provides an in-game reason to mix together elements of multiple settings, genres, and ideas. Our high school group rotated GM duties and having Spelljammer allowed us to add and remove just about anything. The overpowered laser pistols from the last adventure?
They only work on the planet you just left. Considering that you are being chased by half of all organized crime on that world, going back isn't really an option. You might get a decent price from the Mintakans† - they tend to be less discerning than the Arcane.
Then there was the third group of Gith followers, the Githxaren. They were neutral in the struggle between Githyanki and Githzarei. They drifted in and out of the Ethereal Plane quietly building a peaceful homeworld away from the mind flayers and the other Gith. Since they were based off the Githyanki and Githzerai in Fiend Folio, they basically had the same limitations, but they also had a druid-paladin type of class that I have since lost to time and several moves.
On the home world where all the PCs began before heading off into space,there was one mech from Battletech, an ancient Atlas that had just enough energy to launch either one shot of the PPC or a short-range missile. I put it there to battle the Tarrasque if the players managed to wake it up. (They never did.)
I had my minotaur cleric from Krynn, but others had human fighters, thieves and magic-users that wouldn't have raised an eyebrow. There were worlds with all sorts of odds and sods from all over. Yes, I had the giant space hamsters, gnomish ships (though I claimed that they were from Krynnspace). The name Iuz was thrown around a lot as if Greyspace didn't have enough issues with an evil demigod. There were time travelers, grey aliens and even Voyager I (though the PCs never found it).
If that wasn't enough, then a trip through the Phologiston would bring you to Space 1889 (Thanks Polyhedron 73-74!). There were zat (SJR4 Practical Planetology) found here and there in every sphere. You could find scro and nilbogs just about everywhere, whilst orcs and goblins were pretty rare. Did I happen to mention the phraints and the planet of the gorilla bears?
We Get It: Spelljammer is Gonzo
Well there is that, but I didn't always run it that way. Some adventures were cargo missions and many were just exploratory. Things really didn't get really weird until we all went to the Outer Planes. The point is that I felt free to add things I liked, but I had a convenient way to remove things from play if I needed to. In some ways, it was like continuity in the Doctor Who universe - The Great Paradox is that there is no Great Paradox.
I overthink settings quite a bit. Recently I posted about generating a reason for Clerics to use maces instead of edged weapons that required a rewrite of ancient history where mankind skipped the bronze age due to a lack of copper. This further necessitated that the universe uses tin pieces and that copper pieces are used the way most people use silver pieces. Once the iron age came...
I can get lost in all of that because I want something to be internally consistent and passingly logical.
With Spelljammer, elements of the universe simply exist without explanation or reason. You could try, but why bother?
Who built the great Spelljammer ship? While you spend time in the Library at the Nexus of the Multiverse answering that question, we're just going to throw a lasso around it and see if we can steal it.
Why do the Arcane sell Spelljamming Helms, but not other advanced technology? You still buy from them? Those has-beens are outclassed at every turn by the Mintakans†. Besides, the Mintakans are more fun at a party.
So gravity goes to the center of the ship in most spheres, but there is no gravity in others. What's up with that? Your biggest concern is finding a sphere where air envelopes don't work. We got plants all over the ship just in case, but that only buys us 15 minutes of air. If we find one of those spheres, I think we'll throw you off the ship first.
With the ability to move in three dimensions, how can a ballista be an effective weapon in ship to ship combat? You just took 8 hull points of damage from an accelerator. The scro have magic missile guns on their ship. Shut up and load your cannon. We can't take another hit like that one.
How can making things in a planet-sized forge move a ship? Can you really take a giant hamster seriously? You didn't mention that all ships are shaped like animals, except a human Tradesman ship. Don't get those, they're Maneuverability Class C on a really good day.
The setting wasn't perfect - I disliked that mages were the only pilots. Even with that, the game was fun. Crawling around in dungeons are fun, too. I like a good hexcrawl and trips to unknown planes, too. Spelljammer represented to me an escape from Gygaxian Naturalism and Jacqauy-ed dungeons. After a session or two of Spelljammer, even if there were no laughs, I found it easier to enjoy the "real world" of a more typical fantasy world.
So there you are, I talked about Spelljammer for an entire post without drifting off into the ether. Next time, I get to talk about the d12. Oh the d12...
† I don't remember the name of the trading rivals to the Arcane. Since they were loosely based on the Orion Free Traders in Star Trek, I took the name of one of the stars in the Orion constellation as an easy way to reference them.