Now that the 5th Edition Spelljammer: Adventures in Space has been out for four days and I spent the weekend reading all three books, I thought I would revisit my thoughts from above (and elsewhere over the years in these forums): I was wrong: the Astral Sea works brilliantly. As someone who purchased the 1989 box set the day it came out and ran adventures using Spelljammer for three years or so back in the day and who has occasionally gone back to it, I will say I really like the new set. I think the emphasis is in the right place, the use of the Astral Plane works much better than I thought it would, and any DM who was using the crystal sphere/Phlogiston cosmology could continue to use it with nary an interruption (e.g. that system is bounded by a Crystal sphere and has a Phlogiston barrier between it and other systems and, eventually, the Astral Sea).
I really appreciated how consistent Perkins and the other editors and writers were with the original set (examples: using the same speeds and layouts as the original ships with hardly a change, using the same history of the Rock of Bral with minor changes, keeping the spirit of the set, but streamlining some of the original mechanics all in the name of a better play experience, even using some of the same text as the original set in some places).
A few items I am missing from the new set are the alternative helms/power sources like the dwarven forges powering the Citadels and the orbus powering the beholder ships. I thought those were cool. I also miss the elvish Armada ships...they reminded me of huge star destroyers. Also, I am not sure why they did not offer the Crown of Stars as a magic item. I do not miss the Lizardmen in space. I do not understand why mind flayers are not part of the random encounter table in Boo's Astral Menagerie (and the DM's screen). But, really the set is great. I like the Astral Sea revision. I am happy to have been proven wrong and I am delighted that the new set is available as I always loved Jeff Grubb's approach to space fantasy (which is why I quote him from Concordance of Arcane Space in the quote under my avatar).