I had two very different experiences.
My first start-up, NanoGram Devices, was about clear whitespace with a single defined idea. I saw an abandoned technology, and white space in a soon-to-be-fast-growing market. I licensed the technology, recruited the team with my co-founder, raised the VC capital, and sold it to our top competitor for $50m in 12.5 months.
So classic “whitespace” start-up. Saw it clearly, people said it couldn’t be done, didn’t really know what I was doing, ignored them, did it anyway.
The second time, EchoSign, it was more about a theme, and a team. We wanted to (x) do a start-up together, and (y) something in the theme of “Flickr for your Docs.” We bandied about a bunch of ideas, and ultimately settled on this one as the best fit for the team, and went for it.
So FWIW I’ve founded both types of start-ups … Whitespace, and Let’s Do a Start-Up Together and Then Figure Out Which One Exactly To Do.
I think both can work.