So what's the difference between a vertical and "gap-filling"?
If Google didn't have product search, or blog search, and someone went and did that, would they be gap-filling or going after a vertical? Seems to me they're one and the same.
The OP quoted Spolsky mentioning “a feature... for dentists”. That’s one example of a vertical market.
My last employer made a search engine specialized for geography: government agencies and energy companies (two other vertical markets) paid beaucoup bucks for it. (Not beaucoup enough to satisfy the VCs, alas, but the product itself is still being sold by another company.)
If Google didn't have product search, or blog search, and someone went and did that, would they be gap-filling or going after a vertical? Seems to me they're one and the same.