I actually disagree. I think getting users is a subproblem of creating a MVP (Minimally Viable Product), which is incredibly hard in these spaces. You can't get users unless you have an MVP. But what if your MVP takes 1000 person years to create? (That's not the case for email or a travel web site, necessarily, but certainly is the case for an airline reservation system... or a web browser.)
Remember Vonage? All they had to do was offer an MVP for voice phone. But it turns out that if a customer picks up the phone once a month and doesn't get a dial tone and a clear connection, they're going to abandon you.
The Lean Startup wisdom about focusing on an MVP and getting to market fit is absolutely right. The issue is: what exactly is the MVP for your space? Not all MVPs are equally easy to create. Try creating an MVP for a nuclear reactor, for example. Well understood tech? Yes. Easy? No.
