Sure, the real estate seller may not know about apps, but they do know what works and what doesn't work in the real estate market, what the potential legal issues may be, and where the pain points are. I've never worked in real estate, so I don't.

I suspect that's why so many pure-developer run startups are either things that only interest other developers (project management tools and the like) or things that "everybody" knows about, e.g. yet another Twitter client or social networking thingamajig. There's a ton of cash in vertical markets we don't touch because we don't know the right people in those industries to partner with.

The successful businesses I've worked in(startups that went on to achieve success, or already successful small to medium sized businesses) have all been in vertical markets - from forestry to tourism to retail marketing. You couldn't have built these on developer savvy alone. That may go against the HN philosophy that software developers are the be all and end all of startups, but the reality is that most successful businesses need teams of people of different backgrounds.