However, the more realistic view is that app.net will remain a niche service, but one that suits techies very well. So, yes, John Deere will corner the market in tractors, but never branch out to cars.
What'll drive adoption? The developer incentive program. This also answers your John Deere example... There will be a lot of offsprings from App.net. In a rudimentary tone - a fb-clone, an instagram clone, comments systems (bit.ly/qbdebut)etc...
I don't doubt that there will be lots of offsprings from app.net, I'm just dubious about whether they'll gain any traction. "You only have to use one username/password!" isn't enough to get people to spend $5 a month.
It is, in my opinion, the number one pain point on the web today. It affects users of all technical levels and the more you have invested in the web the worse it gets.
I don't disagree that people might pay for it, but why would it by App.net I'd pay? Why would they even pay though? You can trust the App.net developers? No more than those listed, realistically.
Disclaimer: I don't know much about building websites and I don't understand why OpenID hasn't got more traction.
Your vehicle analogy completely falls apart and really on Hacker News do we have to simplify tech products down to car analogies…