There were some good things about Eberron and some bad things. It is a setting that I feel had a lot of brilliant potential, but also some remarkably bad design decisions.
The Good
* The level of technology was superior to most settings, but not sci-fi/fantasy-level like Planescape or Spelljammer, but more like a fantastical version of late 1800s to early 1900s steampunk style.
* Vast cities with production bases and towering buildings to use as locations.
* Potential for a highly political bent to the adventures, far more so than other settings as everyone has various connections and everyone is vying for power rather than really clear ruling dynasties.
* Proper implementation of goblinoids, lizardfolk, orcs, rakasha, and sahuagin in a way most settings have utterly failed to do. Instead of simply tacking them on as "this is a form a 'monster' can take and who cares why they look this way or why they are here", there was thought put into implementing them in the world.
All of these allow for adventures that couldn't be done in other settings.
The Bad
* Tearing all horror elements out of vampires, werewolves, golems, dopplegangers and so on, reducing them to just supped up normal peoples and offering lesser versions of them all as typical PC races that everyone would see regularly wandering the streets. When "half-werewolf" is just the most common of all sorts of people you could run into, no longer can you sell the Werewolf off as a berserking rage-filled beast that is likely to harm anyone near it, including loved ones, and spread its disease and madness unchecked without you much having any power to stop it-- only able to make gut-wrenching decisions about how to stop the disease from spreading. Nor can golems be a demonstration of hubris coming back to punish the creator and all who might inadvertently have committed no crime but to be between the creation and the one who put it into motion. The fact that horror is basically
* Giving magical mutant superpowers to all members of the PHB races and only members of the PHB races. Although certainly some drama could ensue over the politics of whom people are allowed to marry or breed with to keep control of the marks "in the family", since that is an angle no one was willing to pursue it was a bad idea to introduce the concept as something reliable and predictable with only the "special" races having them and being able to be so certain about which marks would ever show up on which races. It would have been better if not only the PCs could have had them, but it was entirely possible for an Orc to be born that had a mark allowing her to create and spread fire and use that ability to rally an army of savages and slaves behind her or a lowly Kobold who was born with a mark that allowed him to charm people and abuses it to create a shadowy underground crime organization while becoming a hidden puppetmaster whose identity can only be grasped at. Limiting the dragonmarks to only PHB races and being prescriptive about who can have what really undermines creating mysterious or dangerous adversaries.
* Setting the nations too far apart and making them too monocultural. By the book, it is sort of lunacy that the typical adventuring party would even come together. Each and every nation has their proscribed people with clear borders between everything. This makes it so that all the conflict can really only happen at the borders. It is far better to be working with multicultural layered cities within whom power can shift between one people to another almost overnight. It would bring the conflict and adventure more to the unique features of the setting and set things up so that the PCs' actions could have great rippling effects without them even necessarily intending to, much less requiring them to overthrow whole nations to make any mark.
* Trying to stuff everything from every setting that ever existed into the setting. While some elements were put in there with care, a lot wasn't. I think the height of this is the idea of a "nation of monsters" as in every imaginable thing that doesn't get along with humans and lies outside the well defined space can apparently peacefully cohabitate in the same place. It is just an utterly bad concept. This also carries through to a major diffusion of Eberron unique core elements that I complimented it on. I have played two Eberron games-- an RTS one and an MMORPG one... and neither of them demonstrated the kind of semi-modern tech nor political structures nor sprawling cities... the first was just three factions on a deserted continent trying to gather crystals so they could kill the other two factions and claim the continent (and one of those factions was all Lizardfolk) while the other was just... kind of generic and dull for the most part. (Granted, I didn't push myself for even a whole week, so I am sure I got to see very little of the content)
Basically, overall while Eberron has a few elements in it that if focused on could totally create a unique experience that could only be had in a setting with its unique features and open up D&D to a totally different sort of game style than it had previously lent itself to exploring.
However, instead it gets laden down with all these factors that instead mean it generally gets used as a very bland, gamist, unsuspenseful, unchallenging setting with empowered standard heroes, lackluster or simply anemic antagonists in a world that is just inevitably completely stable, stale and dry as nothing can change and the heroes are just lesser trainees of the forces that have all the power in the world. It really ends up feeling very much like the pop MMORPG of settings with everything set to easy mode and nothing having any impact.
It really seems to me like the best approach to take with Eberron would be to remove as many of the bad elements as possible, not necessarily altering the world geography, but certainly altering a lot of what is going on in the regions by having them all be highly multiracial and instead divided by what sort of climate and real life cultural apparel tends to be reflected in each region and what sort of organizations are vying for power within it... in fact, maybe just start by focusing on one particular major metropolis instead of the whole continent. Make the really monstrous monsters into true horrors again and have them operating in the shadows right below the surface. Shifters, Changelings and the like should need to hide their true natures, lest a bounty be put on their heads. Warforged should be regarded as soulless, emotionless misbehaving machines with no rights by the majority of the populace as they were built for a very specific purpose, served their purpose and now just serve as reminders of an old war and the regrettable lengths people needed to go to in order to win it.
Make Dragonmarks far less reliable and far more random-- in fact, maybe consider them being something that only remarkable NPCs are likely to have! It should be something only high ranking members of "nobility" or "mafiaso" (as though there is a clear distinction besides who happens to be running things publicly) are supposed to bear aside from those that tend to appear randomly on people who generally tend to abuse their edge to rise to power if not hunted down and culled by one of the families quickly. Even if a player has one, they should be considered "marked for death" and need to hide it and be careful when they use its power.
Just so much could be done to turn the setting around and refocus it into a really distinct and unique experience. But that would mean reworking a lot of stuff I am sure its current fans feel quite attached to.
Of course, I think if one simply posited that a long time has passed since the 3E/4E incarnations and this version of the setting was simply hyper focused on one particular metropolis, it would go a long way towards making those changes more palatable. Fans of the "no conflict, super hero characters" version could reason that a lot of these factors might simply be true for other places in the world.
