The main difference between Brat and Gruden from my limited knowledge of both offenses:
Brat had his receivers run designed routes. And in those routes, the receiver was supposed to look at what defense he saw in front of him and adjust his route accordingly. That was it. 3 different guys running 3 distinct route trees. Very little mix and match between the receivers routes and how defenses would react to each simultaneously.
Gruden on the other hand is calling simple designed routes for his receivers. However, though they're simple routes, they all coincide and are timed in that two or more receivers routes are supposed to force the defenders (LB in coverage, FS, etc depending on coverage and play call to that coverage) to make a split second decision where they should go. Any play where multiple defenders aren't on the same page or even an individual defender takes too long to decide, it's a broken coverage and the receiver will either be wide open or the defender will at the very least be a step or two behind.
The latter of the two above is what you expect out of any competent OC. The most simple, but complex WC system is in New England. Their offense just blows my mind every time I watch it.
...or I could have just made all that up.
