More than just Eberron not being traditional D&Desque fantasy, its also a kitchen sink setting. Everything fits somewhere, it was always meant as a ground up D&D setting exclusively. Its part of the reason that Planescape as much of the same aesthetic, because D&D is just weird in terms of what it draws inspiration from. We've got Lovecraftian horrors like Beholders and Mind Flayers in the same setting as Abrhamic fiends and angels as well as pixies, leprechauns, goblins, elves and fourteen types of golems.
Eberron takes all of those things and makes them work together, usually be modifying the base assumptions about where those things come from to at least some degree so they are all linked to the core assumptions of the setting. The setting then gives you excuses about why new monsters can be used: The Mournland, Kyber, extraplanar, mad-artificers, everything to ever explain why something new is there is baked right into the setting! Its the perfect D&D setting in that sense since everything, and anything, that gets published for the game can be explained with complete confidence
True, and because of those purpose built feel, new stuff doesn't feel forced. Things like goliaths, dragonborn, eladrin, etc work just fine without even much explanation, and without feeling forced. I actually think the 4e Eberron books tried too hard to fit things in, rather than just finding places for them, especially with Eladrin. IMO, that wasn't even needed. THey could have just said, Eladrin are just elves. SOme elves, regardless of culture, are a bit more magical, probably as a result of Giantish experiments, long ago.
Or," in recent years, a small number of elves have developed a stronger connection to the magical energies of Eberron, and display almost dragonmark-esque powers as a result. No one knows why, but at least one dragon has been poking around asking questions about these elves, and tensions between Argonnessen and Arenal are running higher than they have in at least a generation as a result. "
But Dragonborn felt like "here's a thing that has always been true, and we just never made note of it.", as did Goliaths.
My only regret for the setting is that there aren't more airships/more non Lathander airships, and other cools things like paperwings from the Abhorsen books.
But the political, cultural and moral complexity of the setting, combined with the practical, scientific approach to magic (in terms of how it's treated in the world) is at least a third of what appeals to me about the setting .That, and I love the idea of playing an Indiana Jones and/or hardboiled PI noir inspired campaign in DnD.
Right now, my wife and some friends and I have an ongoing Eberron campaign where I play a Shadar-kai Inquisitive who spent a few decades in a fighting arenas in a pseudo Persian area of Droam before winning his freedom, and eventually ended up in Sharn and found work investigating things, etc. My wife plays a disgraced Knight of the Ebon Skull with anger issues who is learning from my character how to control her emotions. It ends up very much a mix of old noir pulp stories, gaslight fantasy, and Dresden Files. And I'm not sure how well such a campaign could work in FR, for instance.