1) Superb leadership - both at the top (Ton) but we also have a great deal of leadership throughout the team.
2) Good communication skills - we don't do flame wars, we hold a meeting once a week to plan technical goals and keep everyone on the same page.
3) Market related - When Blender first went open source, there weren't any decent tools that did everything Blender did for much under 10,000$. So unless one was willing to pirate, it was Blender or a huge up front cost, or a tool inferior to Blender that was still 500$+.
4) Sexiness - Working on 3D is something most programmers grew up dreaming about; simulation tools are a sexy, high prestige, and exciting part of programming.
The confluence of those factors have lead to Blenders success.
Note that the post I'm responding to actually underestimates our dev base by quite a bit, since we moved most of our scripts to a seperate svn which the main tree pulls from.