I've heard a large amount of rumors concerning Blizzard's new MMO over the last couple years and very few facts. What little information we do have is based on vague developer interviews and logical assumptions. It's frustrating not to have any details, but I can understand why Blizzard is holding onto secrecy for as long as it can. The premature advertising blitz of several recent games has been blamed for creating an environment of hyperbole which raises player expectations too high. These games tend to have an impressive amount of initial box sales, but disappointing subscription numbers. Another reason Blizzard might be keeping quiet is that there are a large number of World of Warcraft clones in existence. You could be pretty sure that Blizzard's new MMO will have a similar wave of poorly implemented imitators.
2) It's not based on an existing Blizzard license
3) It will be a very different game from World of Warcraft