Almost every MMO that has come out in this past WoW-centric era has failed in every meaningful way. Most of those have tried, and failed, to emulate the class and questing structure that WoW has spent it's lifetime perfecting. Lets face it: If you want to play an MMO where you pick a race, pick a class, and pick up every quest available to kill boars- rinse, repeat, level cap-, you're already playing, or have played and quit WoW, and don't want to play WoW lite. If a TES MMO has any chance of being even remotely successful it has to bring something new and innovative to the table. Something that isn't just setting or a few gimmicks to try and reel in a few subs. I don't care if this MMO lets me have a pet dog, I don't care if all the quests have voice acting, and I could care less if the game was made by James Cameron and set in 3D. The game, and the gameplay the entire way through, needs to be fun and refreshing.
Ever since the death of Star Wars Galaxies (Pre-CU) I've been dying to see another triple-A MMO title that is sandbox based, and I could not think of an MMO with a better basis to attempt this than a TES MMO. The Elder Scrolls has always been about open world, make your own class, adventures; and I see no reason this MMO should be any different. Classes limit you. Levels limit you. A TES MMO should not have such self imposed limits like this. Let the player, and thus the character, be free to be what it wants to be. Getting rid of this rigid class structure will do wonders to make the game replayable and more open feeling. No more picking an archer, only to have to reroll to become a warrior. Why not be both? Hybridization should be encouraged.
There are so many intricate factors that weigh into whether an MMO is good or not to begin to even touch the surface in this post. Questing should not be grind. The game should not "start" once you hit the level cap (I'd prefer no levels nor cap) because it always ends up with the player hitting a content cap in the game, and then you're stuck on farm status until the next update. PvP needs to be a major part of the game and needs to have viable rewards and appropriate motivators to participate (IE game world consequences for losses). Crafting cannot just be tacked on and the gear available from it should be equivalent to any gear obtainable in any other fashion, but balanced around cost and material requirements. The game needs to be fleshed out and robust in several areas and should not remain static. Game worlds that are too static are prone to getting an MMO-pedia, where every possible detail of the world can be written out on some website, telling you exactly where, how, and what to do at any possible point in the game, which completely kills any amount of mystery or immersion. The game ceases to be a game and instead turns into a set of mathematical input-output equations where money+time=desired outcome rather than fun.
Of course this is all just my opinion, but as someone who's played quite a few MMO's in his day, the WoW themepark trope has been played out. It's time for a true next gen MMO. Feel free to disagree/discuss/whatever. I hope once details are announced this doesn't turn out to be another WoW clone...