For your startup, are you more interested in a horizontal (one function across many industries like word processing or collaboration) market or a vertical (multi-functional in one niche like medical claims processing, hair salon, or architecture) market?
As far as I understand, b2b/b2c is orthogonal to horizontal/vertical. But HN polls seem to allow voting for multiple alternatives, so b2b/b2c could be added without harm.
(Edit: now I see that's exactly what you were suggesting. Sorry.)
I'm not sure any of these are a healthy approach to build something 'useful'. I'm sure once you have built a useful product you could later say that it was 'Horizontal' or 'Vertical' etc, but to build a product strategy based on these concepts is to 'rule out' a potential market before you've even started...
You've got to have some kind of strategy to begin with though, don't you? I'd assume you would have an idea of who you were targeting with your application, even if that target audience changes.
Not only that, but vertical has a better chance of even making it past the short-term. With a vertical, it's much easier to find a pain point that needs to be solved, build it, and focus your marketing efforts.
That's an interesting one, Historically the opposite has happened.
Build something for a niche, sell into that niche, then expand outside the niche by adapting the product to other uses once you have sufficient strength to do that.
It takes a lot more in terms of resources to develop for a niche than for a 'horizontal' market, the marketing expenses are also a multiple.
My opinion is that building horizontal lets you enter different verticals more quickly (and exit them as well). Or perhaps get your toes wet in many verticals to see what's nice and warm before you jump in.
That is also interesting, no need to do another poll for that.