Spelljammer had some neat ideas, but did a few things I didn't quite like too.
I didn't really like the flammable phlogiston and how every solar system was inside a crystal sphere. If they made all these efforts to have earth, water, fire and air worlds that could be spherical or cube-shaped or flat or other things that violated modern day physics, than they should have gave variety to how different solar systems were arranged. They should have had it so that some regions of space functioned differently from others. Crystal spheres and the phlogiston in some places, open space in others, seas and rivers of 'dark matter' in some places, and other ideas.
I didn't like the Rock of Bral being the "drop in Greyspace, Krynnspace or Realmspace". Yes I would have actually liked more of a disconnect from those settings. I think they eventually came up with another solar system for Bral.
Now on the other hand I liked many of the monsters from SJ, old and new. The Illithid Empire, Neogi, Beholder Nations were all neat ideas. And I even liked other things like the K'rrr, the backstory on the mysterious Juna (I thought they were so neat I wrote an
article linking them to the Tiraphegs and Synads), the arcanes/mercanes, and even the Ziggy Stardust (or whatever)-inspired Reigar. Now I think that Giff are sort of goofy being hippo-men, but hippos are very dangerous animals.