Nice suggestion! Exits does indeed need improvement.
Myself, I have been playing with the thought of using icons;
Where you have a set of icons for things like: north, southeast, up (stairs), down (ladder), exit, entrance, etc. And for each exit, you can select one suitable icon.
Further more, by letting the client give semantical meaning to icons (such as icon north having the semantical meaning of going north), you could even draw them up in a star compass like navigation GUI on the client.
But your suggestion to group exits by categories (and color coding them), is also a really nice idea that could further give meaning and clarity to exits. And the ideas are not mutually exclusive 