I think the biggest achievement of Skyrim (and the other Bethesda open world RPGs - at least Oblivion and Fallout 3) is the ability to immerse the player in their world. The feeling of adventure from escaping the prison in Oblivion, the vault in Fallout, or the execution in Skyrim is a unique experience to me that I haven't gotten from other open world games, even those I prefer above the output of Bethesda (Witcher, BotW, Elden Ring, etc.).
I don't think combat and moment-to-moment gameplay is the strength of these games, It's the first person perspective, seeing a settlement over the horizon, picking a house clean of silverware to earn a quick buck, breaking into a castle for the heck of it. The actual implementation of these things always feels kind of jank, but I think it coalesces into a very immersive experience that I don't think any other studio has managed to beat. I can't put a finger on what exactly creates this feeling, but I imagine that it is both hard and expensive to recreate, which is why I think most open world RPGs aim for something different.
That's also why I hope Starfield (and TES VI) doesn't simply aim to create a "bigger and betterer" world, making the game bigger horizontally. I think they should make the game bigger vertically, doubling down on all the complex systems interacting across each other in an already huge open world. You could argue that the ability to steal a ladle with no actual value from the dining table of a local citizen doesn't add much to the gameplay. But the compounded existence of all the minor systems and details is what I think is part of the Bethesda Open World MagicTM.