While I've only played Skyrim and some of Oblivion- and, so, I'm no long-time fan of Bethesda's games-, I've found certain features of these games that I've very much enjoyed. Elder Scrolls games have a distinct appeal as opposed to MMOs (say, WoW, a game that I, http://us.battle.net/wow/en/game/mists-of-pandaria/, enjoyed).
-First, Elder Scrolls Games have an evolving, character-based story. While yes, radiant AI and a larger world mean that things happen outside the players control and the world is continually changing, the player character himself is the primary force behind world-changing. I don't think this could be as effectively handled in an MMO as with a single-player game. I realize that an MMO would take place in an entirely different game than Skyrim, so they could potentially work around this, but think of:
Spoiler The Battle for Whiterun, a quest very much enjoyed by the community. While I realize WoW and other MMOs have implemented "phasing" to change the world according to a player's actions, this really only works well where quests are linear (and player choice isn't involved). So while Bethesda could conceivably eliminate the presence of a building or character following these events, to create to alternate universes in which Stormcloaks or Imperial now run the city for each player is near-impossible.... not to mention terribly confusing for players.
Also, if each quest can be accessed by each individual player, that detracts from player-based control over the world; in Skyrim, every character would lead the Dark Brotherhood, Thieves Guild, Companions, become Dragonborn, etc... meaning every individual character would be responsible for the feats associated (Guards would address everyone as Dragonborn, etc.... Alduin and other dragons- or Elder Scrolls VI equivelants- would be reduced to "bosses" for community raids, and dungeons would require a squad of men to infiltrate. There's no point to developing a character as Dragonborn, because everyone else will accomplish the same thing). Not even taking into account choice that will inevitably change the world into so many alternate universes, even linear progression (say, main quest line) would be meaningless.
-Second, lag... While I love Skyrim and was/still am impressed by the size of the game (it's massive, good graphics, a huge amount of dialogue- even if we all wish there was more choice involved in dialogue... it's just a huge game), there's already a problem with framerate as data accumulates (especially on Playstation 3). Can you imagine what it would be like to have 10 players sharing the same Skyrim world over varying levels of internet connection? Let alone 10,000 (and more; a WoW server).
That's not even taking into account the aforementioned problem of character choice- how could Bethesda possibly expect to implement every character's individual decisions in Skyrim, all the phasing necessary to allow immersion, and still expect to allow each member of a server a smooth gameplay experience? Plus, phasing out objects and characters due to individual game choices will inevitably clash with other players' immersion; is Alduin there or not? Is Markarth Stormcloak or Imperial territory? Etc., etc.
-Third, scaling. Now, scaling is, on its face, a minor detail for an MMO Elder Scrolls; we could dismiss it and simply say "no scaling", opt for the WoW model, etc. However, this seems to eliminate a great degree of player choice; with no scaling, a player would complete quests based on "zones" (like WoW, where players go to a village, 'pick up' every available quest, finish, then travel to the next village). Nevermind a player's desire to immediately join the Civil War, fight some Forsworn, become a mage, etc... no, the player would have to go to the Jarl, pick up a "kill bandit leader of X cave", and complete 10 times before he reached a level high enough to, say, kill the Ice Wraith necessary to begin as a Stormcloak.
Oh, you'd like to see scaling implemented, then? I'm not sure how, given that level 80 and level 2 characters may choose to fight in the same general vicinity. You'll either make low-level Draugr no challenge for the most powerful characters, or make the game literally impossible for the lower levels, just beginning (and say goodbye to difficulty sliders...).
-Fourth, the annoying min/maxing mentality and grinding. Some people do actually play Elder Scrolls to complete every quest, earn the most gold, unlock everything, get the best gear, etc. Still, there's a fairly large amount of players who roleplay their character, wear weaker or low level gear for the sake of immersion, eat every day, etc. Assuming there's PvP or no scaling, that's gone. Instead of donning your favorite fur or hide armor, blue mage robes, and other crap-gear that looks cool, EVERY player will be required to equip the best possible rare gear that they acquire by questing; every low-level character will wear Elven armor, the high levels will all opt for Daedric, etc. (of course, whatever equivelant would be proposed in Elder Scrolls VI, etc.). Additionally, if you just want that every XP to advance to the next level (in order to unlock your next quest), you will need to "grind" to get there; that is, find some enemies, and kill them for XP. In order to allow this, Bethesda will need to introduce very fast respawn times in the outer (non-dungeon) world... meaning, every 5 minutes, the bandit hold you just cleared will be repopulated (which reminds me... say goodbye to sleeping and waiting in Elder Scrolls VI, and the game would probably have to be played in real time... the latter may not bother most people, but many people do like to sleep in Elder Scrolls).
-PvP? This isn't as much of a "problem" with a new game as a question; how exactly would PvP work? Currently, you're able to attack essentially everyone, which I'd assume would hold true in an MMO Elder Scrolls.... Just hope that, if it comes out, they don't draw some stupid "Faction 1/Faction 2" line during character creation to prevent same-faction PvP.
-Console players lose on this.... a lot. We've already seen that consoles have trouble with Skyrim's huge amount of data; a console simply couldn't handle MMO Elder Scrolls and, even if it could, communicating with other players by text (which is already a bit of a problem... would you have to initiate dialogue like with other NPCs? How would that work) is a problem for console users.