Other than physical strength/maturity and playing experience in general, the biggest thing I think Mac needs to work on is his movement in the pocket. That said, as a rookie, it's already at a pretty high-level so we're picking nits here.
Sometimes he sits too deep in his drop and lets a rusher from the edge get him. You actually saw that play out in real time in the Cleveland game, when on one of the Pats' early offensive drives, Mac was sacked. Later in the game, in almost the same circumstance, he made a much more noticeable/deliberate slide-up into the pocket to avoid an edge rusher, and (if I remember correctly) it resulted in a completed pass the second time around.
As good as Brady is in every regard, his pocket presence has always been in a complete different stratosphere and might be his single best trait (relative to other QBs). He always knows exactly when/where/how to move. He knows when to shuffle slightly left or right, when to take half a step up, when to take a full step back, when to duck, when to bob, weave ... in a way that is just so, so consistently perfect and exacting.
To me, pocket presence and pocket movement are two unique (but interdependent) traits. Some are good at both, some are good at one or the other, and some are bad at both. Brady is good at both. Peyton was bad at both. Eli was good with pocket presence but not pocket movement. Brees was pretty good at both, but strikes me as someone who had better pocket movement than pocket presence.
The good news is that I think Mac's necessary improvements in this regard are almost entirely coachable/learnable, and aren't due to some fundamental lack of ability/talent.
