Second, developer blogs. Give people a chance to chime in, as well. Developer-consumer interaction is key. Don't repeat the Skyrim/PS3 debacle, or that will surely doom this MMO.
Endgame content. Believe me, hardcoe gamers will blow through the leveling in only a matter of days if not hours. You need some good endgame content so those hardcoe gamers can get their fix while the casuals have fun exploring and immersing themselves (I am the latter).
Give people leveling up plenty to do as well. If I have multiple zones I can visit for a few levels, I'm the type that will visit them all as long as they're not terrible (Desolace in WoW comes to mind, or later Stranglethorn Vale).
Class balance. People like to have fun with their particular classes. So make them all work.
And last, something that Bethesda/Zenimax have already mastered: Be EXTREMELY aggressive in the marketing campaign. A great example is the marketing done for Skyrim. It was fantastic. And it got people playing.
All these things are a major reason why WoW has been so successful. If the company follows this, ESO will be just as successful. Don't just rely on the Elder Scrolls label for success. SWTOR did that, and look at the state of their game now.