I believe Eliezer was one of the last SIAI employees to move to Berkeley. My guess is SIAI originally moved from Santa Clara to Berkeley because some SIAI employees had rationalist community friends in Berkeley, and when visiting those friends, they noticed they liked Berkeley better than Santa Clara. (I've lived in both places--IMO Santa Clara is dystopian and suburban, but Berkeley is lively and interesting.)
I don't believe there was significant community buildup in Santa Clara before the move. So maybe the takeaway is to make your HQ a place where people want to live for reasons other than just being part of your community?
In short, I'm wondering what place/group/organisation/activity could do for rationality in Europe what Berkeley does for rationality in the US?
Soon, we'll have LWCW in Berlin, which I hope will be an occasion to do some networking among people who think seriously about developing rationality communities. But in the meantime, let's do some brainstorming.
Important note: in comments to this post, please use only consequentialist language. For example, say "If we decided for the base to be on Malta, then X would happen" instead of "I think it should be in Malta, because..."
Of course, share anything else that you think is relevant to the topic.
Also, see you all in Berlin :)