We got off track on impacts and such, but I still don't think we've answered the original question...
The original point of this thread is that at the time of the KT extinction, there seems to have been relatively little difference between the classic survivors (mammals), and dinosaurs that went extinct.
In order to help the thread along, at this point I would like to narrow the question from "Dinosauria" in general, to specifically Therapoda
In other words...
Some Therapods were warm blooded, Mammals were warm blooded.
Some Therapods were small, Some Mammals were small.
Some Therapods had feathers (down), Mammals had fur (fluff).
Some Therapods ate eggs, Some Mammals ate eggs.
Therapods were robust and diversified, Mammals were robust and diversified.
The differences between the two groups are not dramatic, and yet the result was.
Many creatures from the mammalian line survived, but from the Therapod line, only birds survived. I can perhaps understand why birds survived, because of their unique skills (flight). But that really leaves us with the original question: Why did multiple lines of Therapod vanish, when multiple lines of mammalians, all very similar in characteristics survive?