I've played quite a few MMOs over the years, starting with EverQuest. I've also seen quite a few reasons for them failing leaving me with significant trust issues.
First how do I measure failure. BioWare has stated that SW:TOR is profitable, with seven US Servers after a mass exodus from the game with massive server consolidation and going FTP/PTW. Now I know BioWare isn't going to open their books for me to take a peek, but while they may be writing on the right side of their ledger with black ink, how may zeros are there. Did they plan to make hundreds of dollars on this game?
I've seen quite a few MMOs collapse in this fashion. To be fair some of the things that kill an MMO aren't the fault of the Devs.
Botting/gold farming/goldseller spam. This more than anything else killed Aion. The Devs were warned about this before release and ignored the warning.
Harassment. I quit playing WoW because I was tired of the nonsense that went on in trade chat and having a 12 year old tell me what he did to my mother.
Hacks/exploits. Anyone else enter an 8/8 battleground in SW:TOR and notice 12 players on the other team? Add to this the fact that beta testers of the game discovered these exploits and sold them on their websites instead of reporting them so the holes could be filled in by the Devs.
Twinks/Elitist thinking. I noticed this first in EverQuest, like Heirloom gear in WoW, it produced the unwanted effect that new players were not worthy. If you can't keep new players your game will die.
Great start/mediocre middle to end game. Who can forget how awesome the first 20 levels of Conan were only to segway into text dialog in the higher level zones. Rift and LOTRO suffered from this too.
Nightmare UI. Having so many spells/abilities that you can't keep track of them all with no macro's to help. In PvP you have to focus on your opponent, not your UI.
Using the Nerf Bat to balance PvP. This is why I quit playing GW2. This more than anything frightens me about ESO. Aion tried two factions with a neutral NPC faction. What on Earth makes anyone think that three factions with an NPC faction can possibly work. I think we can expect to see the Nerf Bat early and often.
Emphasis on group play at the expense of solo players. Why I gave up on EverQuest 2. You couldn't go anywhere without a group. This was also a problem with SW:TOR and GW2, with storyline missions that required you to form a group to complete.
Trash classes. In GW2 the top tier raiding classes were; Guardian, Warrior and Mesmer. The rest were trash classes. For example I had a 60 Necro that watched the end of every fight from the floor. The GW2 Devs solved this problem by nerfing the Mesmer and Warrior and I quit playing.
Listening to the loud/obnoxious/angry players. Most of us don't want what they want in their shouting rants on the forums. Still the Devs listen to them and the rest of us vote with our feet and leave.
Well enough I suppose. One last thing, I believe that even now hackers are compiling a list of exploits to determine which race/skill set will dominate PvP. They will sell this information on their web sites and by the time the Devs close those loop holes it will be too late.
Is it too much to expect an MMO that will still be growing a year after launch?
If ESO invests in GMs to monitor the game. Suspend accounts for harassment. Ban accounts for buying items/gold and using exploits. Pay attention to the players who don't shout in the forums. Well this game might just succeed, maybe.
Like I said I have trust issues.