Like so much of category theory writing, it lacks examples. Seriously, point me to examples where category theory is actually useful outside of specific parts of mathematics, and I'd be very happy. By useful, I mean that it allows you to prove or understand something that would otherwise not have been proved (or is much more difficult to prove).
The point isn't so much proving new results, as showing how different results (that you thought looked similar but were distinct from each other) are actually exactly the same result in different settings. One of the things category theory does is tell you whether a result is "deeply meaningful" or not.
For example, I still don't know how the Segre embedding of projective varieties is constructed (my algebraic geometry lecturer laboured hard to try and impart that knowledge, with the result that I thought it was difficult and abstruse); but I now know that it's a product in an appropriate category, so while the construction may be really nontrivial, the object itself is just the same old product which I already know and love. I now know that the difficulty lay in showing that this object exists and has the required properties, rather than the object itself being in some deep moral sense "hard to understand".
