#1 for me is on your list in indirect form:

The dinners with a bunch of equally intensely focused people.

Every single dinner we were exposed to something new that caused us to look at our product differently. I can point at about a dozen features in our product and tell you what interaction they sprung from. Google Analytics support was written while AT a dinner in response to something someone said earlier in the evening. Everything mobile in our product (quite a lot, at this point--alerts, mobile access, etc.) came from discussions with the guys from Tsumobi and Heysan. And the UI is still undergoing a massive transformation that started last year during those dinners.

So, for me, the other founders were the most valuable part of the experience.