Just to be clear, I am now in Silicon Valley after spending a couple of years in Ottawa.
Miss:
1) Close to friends & family
2) Really great activities/camps for younger kids
3) Lots of parks, green space
4) Canadians are really more polite, and the pace is more relaxed.
5) People lead healthier lives, many more people outside (this does not stop me from doing same in SV, but culture is more work and convenience/car oriented).
6) Cost of living
Don't miss:
1) Tech scene is anemic. Tech companies (even the shiny new ones that are moderately successful) are conservative and can be a little arrogant when it comes to hiring. It doesn't matter how capable you are, as an employee you have little leverage.
1.5) Investors don't know much beyond telecommunications.
2) After the valley, Ottawa seems really small.
3) Um yeah, winter (although I surprisingly enjoyed taking my son to hockey on early mornings)
4) Not a lot of ambitious people.
Silicon Valley:
Likes:
1) Much greater depth of talent, not just in new technologies, but in product, strategy, business development, etc.
2) More going on culturally.
3) More wealth.
4) Weather.
5) Environment which seems to germinate most new ideas in tech, probably due to employee mobility which has a cross-pollination effect as they move from org to org.