I skipped over SJ when it first came out, but I did run a Sundered Skies campaign (which degenerated into 'hunt sky whales and buy real estate').
I am now casting about for new settings, and I see that material is being produced again. Is SJ just 'piracy among the stars', or is there some depth to the setting?
Not "just", but that is a supported playstyle in Original Spelljammer.
It also implies that merchanting can be done, and gives construction and combat rules for space.
There are many places to live in Spelljammer, and the namesake herself is a stupendously large space manta... with a city crusted over her like barnacles...
Spelljamming can be used to move parties between Toril (Forgotton Realms, Kara-Tur) , Oerth (Grayhawk), Krynn (DragonLance) and several other settings; it can't get you to most of planescape, nor to Dark Sun's Athas, nor to any of the locations on the Demiplane of Dread (Ravenloft)... but one-way gates to Ravenloft can happen... and should you sail into one, the ship won't get you out.
Note that several traditionally eeevvvviiiilll species are presented as relatively low threat in spelljamming. Neogi and beholder trade ships routinely trade at certain ports. Illithids seek to pirate and eat, and, failing that, trade with, others.
Basically, Classic Spelljammer provides strong ship combat and ship construction rules, a number of species, old and new, that are common in space, common human, demi-human, Neogi, Illithid, and Beholder ships, and exists as a little known reality of the main adventure settings.
One key element is that it drains the helmsman caster of spells. Speed is by what slot is cast.
Encounters with space life can be amusing... most giants are too small, but knock them up a scale, and they can be a space being...
Note that each setting is in a separate crystal sphere, and physics can change between them somewhat. Clerics outside their home sphere can't renew (but usually can use) their higher level spells. Mid-levels are available via "local" gods; low levels are essentially not regulated save by the restrictions on magic for that sphere. Each sphere is at least a solar system in scale, and crossing between spheres can be days to months in the phlogiston, and no flames for the whole of the trip between spheres, or your Air Goes BOOM!
Spelljammer didn't add classes, but did add some NWP's, tho I know that's not going to matter much to you.
The most important elment of classic AD&D spelljammer is the air bubbles and the gravity planes... ships generate a gravity plane fore to aft, larboard to starboard, at the center of mass; Anything above falls down at 1 gee; anything below falls up. Plan cargo accordingly, because once in a bigger item's gravity well, the ship's shuts off. Most life forms don't generate a gravity plane. Cuboid planets pull down to the closest face; circular toward the center.
Also, all things grab an air envelope based upon their size. A human's is good for a few minutes, so if you go overboard it's not instadeath... but they better notice quick. A ship's is good for a week to a couple months... the Spelljammer itself has enough plantlife to keep the city going indefinitely. And presumablyb some longstanding mythals, as well.
Worlds are any combination of the 4 classic elements, some being one, others being 2, 3, or all four, and stars are really large fire and/or positive energy bodies. Worlds come in shapes including all the usual polyhedral dice and spheres. It's fairly well thought out, and the SJ adventures I read were not even planetary, but involved a lot of space travel and the resulting random encounters.
"New"
Spelljammer: Adventures in Space for 5E...
No construction rules. About twice as many ship types as the adventure demands. A number of races; the Hadozee entry created a furor for including them having overturned their enslaver's culture... And note that Hadozee are D&D's Yazirians (from Star Frontiers). The Plasmoids are very close to Star Frontiers Dralasites, but not as thick. Workable combat rules.
No crystal spheres, no phlogiston. A number of subclasses. It does retain the gravity plane.
The speres of wildspace fade out into the shores of the Astral Sea... but they seem to have dropped the flattening of people as they hit the astral plane.
Note that gravity planes in 5E SJ only shut down when in contact with a larger body... who's own plane then asserts. But air envelopes can merge.
And the air bubble defines the limits of the gravity of the item, too. Anything Gargantuan size or smaller are air only, no gravity plane, and have 1 minute of air... so if you drift out of the air of your ship, you don't suffocate for a minute, then suffocate normally. (no vacuum damage.) Larger items hold an atmosphere as thick from its surface as its longest dimension, in roughly the same shape.
it's rules are a bit simplified from its precursor, in addition to omitting construction, random sphere generation, etc...
If you want to work it, either edition can support spacefaring cultures... but the AD&D one is more self-consistent, but both have major physics breaks as a matter of need.
