I'm Canadian, so I'll give you a one off overview. I'm assuming you are American, so revise this if you are not.
If you are high on lifestyle and lean left in your politics and feel weird when there is no snow on Christmas, you'll more than likely love Canada. Nothing up here will compare to the startup scenes in the SV and such, but if you stick with the bigger cities (Toronto, Vancouver, and Montreal) you'll most likely find enough for you to live with. There are other tech centres like Ottawa, Waterloo and Calgary, but these would be smaller, albeit with the pluses of a smaller city as well.
In general, you need to accept snow and cold in the winter. Places like Toronto can be 50/50 and a lot like say, NYC or Chicago, but Montreal and god forbid the Parries are a guaranteed frozen for 4 months of the year at least. The only real respite from this is the BC coast, but instead of snow you will see 4 months of rain.
You might feel taxed a little bit, but that should only last until you realize that those taxes include all those Health care expenses you no longer need to worry about.
You'll need to accept that Canada is very accepting of its immigrants - almost too much so - and it will be expected that you are as well. Strong Christian right views that compose half of the spectrum in the US will often just not be tolerated here. This is probably the biggest difference between the two countries - your religion and beliefs are personal matters. You will quickly learn about all other countries and cultures from Canada, as there are large communities from the East, Europe, and the Middle East that are encouraged to engage and build the country but to also retain their own differences.
Canada can be VERY different depending on what area you choose to live in, so I'd strongly suggest you look at a few areas and then visit first. Montreal and Vancouver may look similar in size and economy, but they are ridiculously different in many more important ways. Much more so than say Boston compared to Seattle. Pay a lot of attention to this.
I've lived in 4 provinces and visited all the provinces and territories spare the Maritimes. I'll reply if you need any more information about a particular area.
Good luck to you.
The tech scene is decent in the big cities with some strong universities and tech centers. The investment community is definitely weaker than the US (not in volume/assets but in talent: the archetypical US VC partner is a former entrepreneur while her Canadian counterpart is more likely to be a former pension fund manager). It has improved over the years though.
A surprising discovery (for me), was that the most leftist province (Quebec) has by far the most support for tech companies: All the usual Canadian programs such as getting 60-80% of your tech expenses back in tax credits but also some really neat stuff such as a "tax holiday" for foreign developers (that would be you: no income tax for almost 5 years). Oh, and one of the lowest corporate tax rates in North America (coming in around 27% right now).
Tech business aside, Canada is a nice place. A lot less of the steep socio-politico-economic gradient that has become so common in the US. Definitely a good place to raise kids.
The winters are unpleasant though if you are used to the Valley (I cry a little bit inside whenever I go back to SF).
As far as tech, it's roughly comparable to the US, just on a different scale (we've got 1/11th the people!) - most of the buzz is centred on a few cities (Toronto, Montreal, Vancouver) but there are good start-ups pretty much everywhere.
Canada is vast, rugged (mostly frozen) landscape and the people reflect it - we're helpful and friendly but enjoy nature and adventure too.
The two big complaints I've had from Americans moving here are: a) our telecom service (cell, TV, etc.) is stupidly expensive b) our liquor-laws are draconian (arguably this is uniquely Ontarian)
Whether Canada is a good place for your start-up depends a great deal on the nature of your business - if you're looking at industry (especially primary industry) there's no better place on Earth to set-up shop.
I'm a Canadian, but was partially educated and have worked a great deal in the US (the coasts, mostly) and have lived in five Canadian cities and visited every province and territory - I grew up in rural BC.
Toronto is cold during the winter, people are friendly, pretty laid back culture, which can be a bad thing when people don't get angry about the BS that goes on here sometimes. Taxes are high, but its probably worth it in the long run.
Canada is the best.
Startup/tech? In my experience, people who say the scene is really hot haven't been exposed to anything else. As far ahead as Toronto and Vancouver are than say Ottawa or Edmonton, Seattle and Austin are as far ahead of Toronto and Vancouver. It isn't even in the same ballpark.
If you do move to Canada, I'd say Vancouver is the only sane bet (which is not where I'm from). First, it puts you relatively close to Seattle, Portland, California and Austin. Second, by Canadian standards, it has a good tech scene. Finally, by world standards, it has a great quality of life (though not a cheap one).
Vancouver is amazing for the lifestyle.