Building up pure "online" teams is ridiculously hard.
I can tell from a project where I was part, that communicating via Teamspeak or Netmeeting (and the like - it was back in 2004) can be very exhausting and contra productive for getting some sort of "team feeling".

And this one was pretty well thought out. A lead developer who already had worked on successful projects, picked specific people for all the different roles to be filled, prototypes, task management, gameplay videos... completely different to "hey let's make a team and the next big MMORPG - who's in?"

It is due to the reasons already mentioned by HeelX and the fact that online conversation simply cannot replace or even out do the classic worker room where you meet all your team members real life.
Yeah there are successful online collaborations, I know - but it for sure doesn't make things easier.

Ask George Pirvu about the AUM project which started great and then it all of sudden disappeared. And he had some pretty good guys on this project.

And hopefully this explains why we manage to be productive when we meet real life at AckCon but the rest of the year we sort of fall asleep (or work hard on other projects)