Is Daggerfall really the most feature-rich? I remember it being the most glitch-rich (if you exclude Battlespire and Redguard). Arena had create wall, break wall, create stairway, etc magic that allowed for crazy navigation of dungeons. Didn't Daggerfall cut some of that?
I had a long post detailing why Daggerfall's not that feature rich until I removed it and went with something more meta, but basically from a raw numbers perspective, Daggerfalls has a lot of features, especially with regards to char and action reputation, but a lot of them just don't work right (the boat, language skills), force the player to grind a lot (the way reputation works), or just offer essentially pointless options (you can tweak a lot with enchantments but it's kind of pointless).http://www.uesp.net/wiki/Daggerfall:Daggerfall
goes into more depth about the mechanics.
Anyway the particular flavor of open-world RPG gameplay that TES uses is pretty much what Might and Magic came up with well before TES, the key difference being that the M&M games are a lot smaller than Arena/Daggerfall but as I said, that doesn't really count because Daggerfall just repeats/copy-pastes a small amount of unique content over and over again to LOOK big.
Now, I DO think that Daggerfall was the first RPG that used a true 3D, room-over-room engine. iirc Ultima Underworld was still doing the Doom thing, where you had 2D maps that got extruded up. So arguably if you want to put it on the list, that'd be a better reason.