I don't believe Brady is "all that". I think if he played for any other team, we wouldn't even be talking about him now. I may be wrong about that, but look how Cassell and Garrapolo looked in Bill's system. Brady has had the benefit of great coaching and the same system for 15 or more years.
Tom Brady's skills fit the system very well. His ball placement is top-notch, right up there with Drew Brees among the best of the current quarterbacks. His placement is so good that it gives the receivers a chance to get yards after the catch. Plus he has been durable and consistent.
He certainly isn't the athlete of some of the other top quarterbacks and won't buy those third and fourth chances. Then again, he rarely has to in the Patriots offense. Watching New England yesterday I think Brady probably threw 3-4 passes that went more than 10 yards in the air. Most are really an extension of the running game.
It is a good question as to how Brady would fair in other offenses, but there's no doubt he executes New England's offense near perfection.