Eberron is widespread magic. Dark Sun is psionics and forbidden magic. M:tG worlds are all different spins on existing concepts. Greyhawk, Forgotten Realms, and Dragonlance IMHO are way too similar in theme to be brought back.
Again, I disagree with this. The only major similarity is that they all emerged out of the "golden age" of the 70s-80s.
GH and FR are similar in their kitchen-sinkness, with tonal differences that would be expected from the personalities of Gary Gygax and Ed Greenwood. GH is a bit grittier and more influenced by sword & sorcery; FR is more high fantasy, with a lighter feel. Both are meant to be able to run just about any type of play experience.
Dragonlance is more tightly thematic, with a strong post-apocalyptic vibe, a unique pantheon and, of course, centered on dragons. While it can be used in any way a DM wants to, it is more focused in terms of the type of game it was designed for. It was created as a set piece for D&D's first big multi-media event, with a best-selling series of books, so to play DL--at least early on--was to play "kind of like the books." I don't know how old you are, but the Chronicles and Legends were enormously popular when they came out--probably as popular as contemporary series' David Eddings' Belgariad and Raymond Feist's Riftwar.
Krynn itself has a very different quality to FR and GH, as far as the world itself and flavor. Where much of GH and the Realms are based on obvious Earth analogues, with the Realms expanding even further to include a little bit of "everything" (or at least most major cultural groups), Krynn is based around internal themes. There are some obvious Earth analogues (e.g. the plains people of Abanasinia as Native Americans), it doesn't attempt to emulate real-world cultures. There is no "fantasy Egypt" or "fantasy China." There isn't even really "fantasy England."
As I said before, Krynn was designed in a similar fashion as many novelists design settings for their story. Story first, setting second. GH and FR (or at least once TSR took over) was designed as game first, with setting as an environment for stories.
(As an aside, there are some game settings that are designed in a more Tolkienian sense: setting first, stories second, such as Talislanta, which while it was designed as an RPG, more feels like a "boutique setting" made for the pure joy of imagination. Tolkien designed Middle-earth to envision a deeper European mytho-cosmology and to ground his languages in cultural histories, with the story arising out of that)
