Making an MMO has proved to be a challenge to any company that has attempted to do so. Some succeed and some don't. Some thrive and some just exsist in this ever expanding Gaming workd. The most obvious reason of failure within this genre is the under devolpement of a game when released. Such as not having many features, combat being glitchy, zones incomplete, and the biggest. No endgame! Of course most developers promise all features being added at some point, but if the core components that were promised during devolpement are not there during released, can destroy a game in a matter of weeks.
Fans/ players don't tend look at the future when first picking up a game. Instead they look at what the game has in the time there playing it. If it's not worth their time then why play it? That's why I'm here writing and you here reading about what The Elder Scrolls Online (an upcoming mmo) needs to have for success and to flourish for many years.