I think, and this is purely based on experience and opinion and not on any tangible evidence, the biggest issue I've seen with many MMOs is the lack of content. If they were to give us regular content updates to add new quests and new things to do, and then once a year hit us with an expansion and add new areas to explore and new ways to customize our characters such as skills/classes/spells/dieties/etc, then I think it'll stand a good chance of doing well. The IP is solid, they have control over it so they can do what they want, they have a massive fan base salivating over a game that most of them have yet to see, so they're off to a good start. If they can keep that momentum going it'll be a great ride for us and them.
If however, the devs become complacent or are otherwise unable or unwilling to try and meet the voracious appetite of players for content, or the game becomes limited by hardware (like some games are starting to experience with the PS3), or they allow parts of it to stagnate then I think its in trouble.
Overall I hope they look at a lot of the other MMOs out there and what they do right and wrong. There has to be a reason why WoW (even though I never cared for WoW) and UO have been going on so long and still remain subscription based after all.