The supplementary advice (which I guess is left out of this post for brevity but which I've heard PB say in public talks) is that you should work very very hard, both before and during the dev project, to determine what those three key attributes should be. It can change over time.
In the case of Gmail, he spent countless hours sitting next to Google colleagues at their desks, watching them use MS Outlook throughout their work day, and asking them why they were using it the way they were and figuring out what could be improved.
Then when he had a first working version of Gmail, he'd get people to use it and watch intently to figure out how they responded to it, what they liked/disliked, and how it needed to be improved further.
(The first version was mainly just a search box for PB's email inbox. The first thing he was told that needed to be improved was that it should be able to search the user's own inbox instead. But it was still a useful test; they loved the search experience enough to want it for their own inbox.)
Anyway, there was every opportunity to chop and change what the three key attributes should be along the way; the point is to not try and be all things to all people and build every feature anyone ever asks for, lest you end up with your own equivalent of the Homer car.
In the case of Gmail, he spent countless hours sitting next to Google colleagues at their desks, watching them use MS Outlook throughout their work day, and asking them why they were using it the way they were and figuring out what could be improved.
Then when he had a first working version of Gmail, he'd get people to use it and watch intently to figure out how they responded to it, what they liked/disliked, and how it needed to be improved further.
(The first version was mainly just a search box for PB's email inbox. The first thing he was told that needed to be improved was that it should be able to search the user's own inbox instead. But it was still a useful test; they loved the search experience enough to want it for their own inbox.)
Anyway, there was every opportunity to chop and change what the three key attributes should be along the way; the point is to not try and be all things to all people and build every feature anyone ever asks for, lest you end up with your own equivalent of the Homer car.