If there’s a controversial topic between MMORPG players it would be instanced gameplay. Some think it’s great, as it allows the game to convey deeper storylines and to make the player feel like the hero. Some think it’s the bane of the genre, as it essentially makes the world smaller and more fragmented, effectively making the game less a MMO and more a single player RPG.
On the other hand, if there’s a controversial topic between The Elder Scrolls fans when they talk about The Elder Scrolls Online, it would be the fact that it is a MMORPG, while the franchise has always been a bastion of single player gameplay.
During an interview at Game Informer Game Director Matt Firor revealed that instances will be heavily used for the main storyline of the game, that will be completely single player (even if it will most probably require you to still be online and logged into the game).
Whether it will be possible to optionally bring other players to help you or not (as it happens in other games), hasn’t been specified.
You can view the full video interview (that also shares some details on how questing will work) here.