Strange quote. Since the economic incentives require a native token, it's hard to say that anything besides decentralized currency could possibly be the killer app. Decentralized exchanges or stores would probably be number two. It's unknown what number three is. "Smart contracts" might possibly be way down the list even past things like notarization.
More on that subject from some random PhD guy's take on eth that I never heard of from a few days ago:
https://medium.com/@bedeho/why-your-ethereum-project-will-most-likely-fail-d14b6d8f1c7c#.wnguyxb3l
That's wrong.
"In marketing terminology, a killer application (commonly shortened to killer app) is any computer program that is so necessary or desirable that it proves the core value of some larger technology, such as computer hardware, gaming console, software, a programming language, software platform, or an operating system " - wikipedia
The killer app for open source is easy to answer and it was the system software like operating systems, programming language implementations, etc. Open source become so necessary or desirable for that portion of software development that it proved the core value of the methodology. Other uses of it like open source chemistry or accounting software or whatever came along for the ride.
Although, it is possible that had blockchains existed at the time, that too might have been a killer app for open source (the trustlessness requirement for blockchains makes closed source blockchains a complete failure, i.e. open source is necessary).
The killer app for blockchains is currency. The other stuff like notarization, contracts, etc. may come along for the ride, the way dental billing software came along for the ride with PCs (spreadsheets were the killer app).
But the question is hard to answer if you define 'killer app' incorrectly (or at least alternatively) as something that quickly leads to millions or billions of users. That definition is sometimes used in the case of wildly popular video games that drive sales of a gaming console. But in reality, new gaming consoles (even if somewhat improved over previous gaming consoles) aren't different enough from previous ones to even need a true killer app. Virtual reality, by contrast, will need a true killer app, as yet unknown (some suspect porn).