Uhm, its "the number one reason", not "the only one". :b
Anyhoo, in my opinion its the gameworld itself. So much more immersive and believable, glitchy things aside.

Yeah, I can't believe Bethesda didn't do anything about the hovering items, didn't they play the game at all before release? I come across floating weapons and stuff all the time. And why do I fly 200 metres up into the sky when a giant smashes me? Just like all the small details making the game more beautiful, there are many small things that are just annoying.
While the random encounters are pretty cool, I've already gotten the "Redguard woman" one like five times. Getting the same encounters over and over and at the same spot makes the game feel less organic. Also, I don't like the principle of random, unnamed NPCs in that sense. In Oblivion, you'd run into someone in the middle of the woods, and you'd be like "Oh, I remember that guy, it's the one from Skingrad! I wonder what he's doing here" - NPCs truly lived their own lives rather than spawn at your leisure. It's become a matter of "serving the eye" as we call it in Sweden.