The book on Cooks has not changed at all since he's entered the league. Tremendous speed, good hands, excellent ability to track the ball in flight, in addition to limited size and physicality. His lack of size shows up the most in jump ball situations along with being able to beat press coverage consistently.
The press coverage issue can be mitigated with better route running, but as it stands Cooks is very limited in his route running. By that I mean he's not deceptive enough at it. Don't get me wrong, Cooks is very precise in his route running almost mechanical and robotic - but that's the issue. A lot of route running is about deception and leverage. From an outsider's perspective it looks like Cooks is way too reliant on his speed to get separation. Perhaps because of that speed he's never truly had to hone his craft to the subtle nuances of route running like someone such as Julian Edelman, who was not blessed with blazing speed, had to.
Despite his limitations, Cooks because of speed and change of direction should be able to run drag routes, angle routes, and pivot routes far more effectively than he does now. The fact that the Patriots don't often have run these and instead stick with 7-8-9 routes can be looked at in two ways: either don't think he's good enough at them, or they prefer him in the lid-lifter role he's best at.
Either way, he's been effective in New England at things he does well, and not effective at things he does not do well, as expected. I think where he can improve the most, beyond the route running and lack of physicality issues, is his chemistry with Brady. I think even with his obvious weaknesses he could still be much more dangerous when he and Brady both develop a keen trust factor, where Brady can throw to him in spots before Cooks even makes his break in the route rather than waiting to see if he read the coverage the same way as his QB. Cooks has been very productive but I feel like there's been yards left on the field simply because he and Brady aren't always on the same page.