The intial design brief for YAGS was for something simple to design and quick to explain. More recently, expansions to the rules have progressed in terms of me thinking about how to simulate something using the rules.
The term I've used is Cinematic Realism, by which I mean a set of rules based in reality, but allowing some of the heroics seen in the grittier Hollywood action films.
If I see something in a film, or read something in a book, that matches what I'd like to be able to do in an RPG, then I'll start thinking about how best to implement it with the rules. If then rules can't do it, then I'll look at extending or changing the rules if I think that support for this 'feature' would be useful.
The pages listed here describe some of the design decisions that I've made about the rules system.