The space I'm working out of now is Founders Den in SOMA (at Townsend and 3rd). They're a coworking space for experienced entrepreneurs. The former mayor of San Francisco and now-lieutenant governor of California Gavin Newsom works from the space while in SF - that's quite exciting, especially since my desk is right across from his. Founders Den feels relatively quiet compared to the spaces in New York - maybe second-time entrepreneurs know that the road to winning is typing, not talking.
The biggest change from my previous life at Google has been that both in New York and San Francisco, I was able to walk to work rather than having to ride the Google Bus for an hour each way. I suspected that getting those two hours back would be a big change, but the added creativity and productivity has far exceeded my expectations.
I'm still very much in stealth mode and will be for some time. I've brainstormed on my ideas with trusted advisors and potential coworkers, but I'm keeping them close to my chest for now. Unlike reMail, which was a build-a-better-mousetrap play in a niche market, the ideas I'm working on are far more consumer-oriented, and far more competitive.
The contrast between New York and San Francisco couldn't be stronger: In my last week in New York, I was hanging out at Betaworks in the meatpacking district. Walking to work, I would encounter people that looked like models and were talking about fashion. Helmut Lang's design studio is in the same building as Betaworks and you can imagine what kind of people walk in and out of there.
In contrast, on my 2-block commute in South Beach, I walk by nerdlings talking about intricate details of server and mobile technology stacks. Generally, the New York tech scene seems much smaller - and it seems like everyone knows everyone else in the industry. San Francisco has much tech going on - so much that it's hard to interact with people not in the scene.