It’s easy to see why Mystara, Greyhawk, Dragonlance and Forgotten Realms all feel very similar. They all grew organically out of home games over a period of time. It is ever likely they are all a mishmash of fantasy tropes, retired adventurers, city states, ancient empires and their dungeons.
Athas, Planescape, Birthright and Ravenloft are all far more coherent in the vision of that particular setting.
Eberron is unusual in that it was designed and had unique theme, but also has that mish mash feel. Probably because a design goal was to be a one size fits all campaign setting for 3rd edition.
‘Mystara, Greyhawk, Dragonlance and Forgotten Realms all feel very similar.’ To me they are identical and interchangeable.
Indeed, the 5e Forgotten Realms eliminated the sun elf, replaced the moon elf with the Greyhawk high elf, and few seem to even notice.
The settings are identical.
But Planescape is also the 5e Forgotten Realms. Its cosmology now becomes regional settings within the Forgotten Realms setting. Arborea is a regional setting, like Kara Tur. The gods that populate the great wheel, are the same gods that populate the clerics. To me it feels like the same setting. Being in the material plane or in the great wheel is like being outside looking at the castle versus being inside the castle. It is the same setting.
Eberron is a truly different setting. Its design is to use all official mechanics but with significantly different flavor. It has a different cosmology and feels different.
Dark Sun feels different. Again because its cosmology feels different. It has much that I like, psionics, nontheistic clerics, and so on. Unfortunately, I dont get into the post-apocalyptic genre. To me it feels oppressive and ... luddite. Maybe if Dark Sun was more like points of light, where there are ‘oases’ with positive influence and high magic, I could get more into it. Or at least give me the tools to create these oases. If Dark Sun is strictly unrelated to the Forgotten Realms multiverse, then the undesirable polytheism lacks existence anywhere.
Regardless of my complex feelings about Dark Sun, it is a setting that feels different from Forgotten Realms.
Significantly, it is the cosmological backdrop that makes Dark Sun ‘feel’ different.