TES in the traditional sense has been single-player. The player has the entire game world to personally explore and manipulate (within the scope of the game mechanics) however he or she wants. They cannot ruin anyone's gaming experience but their own through their actions, and if the screw something up, well, that's what saved games are for.
In an MMO world, at least in terms of what MMOs have become thanks to the narrow-mindedness of the industry and how it has perceived the fluke that is World of Warcraft's success, there is no personal experience. You play what the devs sell you and that is it. Nothing you do can really have an effect on the gameworld, because if it did, then a bunch of other people would have a tantrum and ride their self-entitlement wagon down the developers throats until they nerf the game.
Until the MMO development industry actually grows a pair of balls and actually decides to become innovative again, we won't see an MMO that offers the kind of freedom that the core series of Elder Scrolls games offers, and what the core TES fan longs for. It's not just the main quest or the profession guild quests that encourage a TES fan to spend THOUSANDS of hours playing in a single player environment. It's the ability to download and create mods that keep the gameplay experience changing. That level of modification will not be possible in TESO.
The faction thing, which does lend itself well to the PvP aspect, also serves as a straightjacket for those who are not interested in PvP. If I want to play a High Elf and explore Skyrim, then I must wait until I am level 50 to see what someone playing as a Nord can see at level 1. These sorts of invisible barriers to personal character development are off-putting to a lot of core-series TES fans who have not been so restricted in their choices of characters for FIVE successful TES games now.
These are the things that make me as a core-series TES fan a little disappointed in the direction they have indicated they are going with this game.
However, I am also an MMO player, so I can accept the decisions. But I am an MMO player who cut his MMO teeth on an MMO that has yet to have a true successor because the industry has shoved its collective nose so far up Blizzard's backside that they cannot see that what Blizzard managed to succeed at, everyone so far that has tried to duplicate it has fallen flat on their face.
And also, the core-series TES fan is not used to having to buy a TES game and then keep paying for it every month thereafter. They may buy DLC for it, but it is not mandatory to keep playing. So the payment model is off-putting. That's all I'm going to say about that...
Based on what has been revealed by ZOS at the conventions and such, TESO has a lot of things going for it that will make it feel like a TES experience. But for many core-series TES fans, I think that the gulf between it feeling like a TES experience and actually being a TES experience may just seem a bit too wide...
Time will tell.