Washington is a great team, and Trotz a great coach.
That said, they lost because of exactly what a number of us said would prove to be the difference. Speed up and down the lineup and depth. Simply put, Trotz could not strap on skates and improve that bottom six. He resorted, by admission, to double shifting his stars rather than play the bottom six. So while the Pens had Cullen's line in the defensive zone taking faceoffs, Backstrom was double shifted, but Trotz dared not do the same thing back with his fourth line.
When you have two highly skilled teams that is enough to be the difference.
It does not guarantee the win, but if these teams played one another a hundred times in a series the Pens would win at least eighty of them. That has nothing to do with the coaches, and everything to do with the hand Trotz and the Caps dealt themselves.