The problem isn't Kant's Categorical Imperative, the problem is that he was sometimes incorrect about what it implies.

The problem with the Categorical Imperative is that it is sufficiently vague that it implies anything you want it to. You can (almost?) always make the "maxim" of your action specific enough to make your action permissible, for example:

I want to kill my professor for giving me a bad grade, so here's my maxim: If you were born on November 1, 1985, are white, have short brown hair, are wearing a black Tool t-shirt and Simpson's pajama pants, and got a D in your world lit class due to attendance despite acing the tests, papers, and finals, you can kill your professor.

Can this be willed as a universal law without contradiction? I certainly can't find a contradiction.

I remember in my advanced logic class, taught by the philosophy department, a latter section of the book formalized the golden rule into a logical system, i.e., do unto others as you would have them do unto you in the same situation. In other words, be consistent. I never worked through that chapter, but I read through the setup and the whole system suffered from a vagueness similar to Kant's: when does a situation count as "the same?" As far as I could tell, everything was moral because no two real life situations could be the same - surely something in the universe moved somewhere. Maybe just an atom.

Btw, yes I really did get a D in world lit because of attendance, and no, I'm not really that upset about it. It was a couple of years ago, after all.