Relevant HN thread. Both the SF startups I've worked for have/had free meals, flexible work hours, on-premise fun like climbing walls, table tennis, foosball, etc., egalitarian laid-back work environments, and so on. In terms of technology you're working with, I'd guess that you're probably more likely to work with something relatively newer and sexier like Hadoop, Ruby on Rails, or node.js here in SF than something like Java. I don't know what you work with in France. In terms of whether the work is interesting... well, that depends on the startup.

How will this affect my relationship with my SO?

That's a tougher one... supposedly the dating scene is relatively bad for men in SF, but I only just moved here so I don't have much firsthand experience. I don't know what your SO's visa options would be. I assume she's not a programmer? If she is, maybe she could apply for a visa too? I don't know how you guys feel about gaming the US visa system by getting married?

Less critically, should I bring my Cello, or should I buy another one? What about the rest of my stuff?

Figure out how much it's worth to you and how long you'd have to work here in order to buy equivalents for all of it or things that made you equivalently happy with your extra salary?

Also, I don't think I value money all that much. I mainly care about the sense of security it provides. If I were guaranteed half of what I currently make to work at home on the computer science research that I want to do, I would take it.

Do you have any interest in effective altruism?

Indefinitely postponing such a big decision is so easy.

Well, you can certainly postpone it until we learn what kind of immigration reform, if any, passes. Even then, I think it would only start to take effect at the start of 2014 (but I really have no clue).