Vanguard was one of the most anticipated MMORPGs to come since EverQuest, but it failed.

Here is the founder, Brad McQuad's blog, that suggests why.

Get everything in writing, don't stop at verbal agreements.

All-star people can have all-star egos (and don't work well in a team)

Don't assume your past success will happen again (EverQuest vs Vanguard)

If you are going next-generation, make sure it is the difference between 2D and 3D (eg: Ultima Online was 2D, EverQuest was 3D)

Design for change (eg: allow for worlds to shrink)