If I change the Media Kind option on a track to Audiobook, what does effects does that have on my iTunes library? How does it change the presentation and functionality of the track?
Jesse Hollington wrote a great guide to Audiobook handling "The Complete Guide to iTunes Books, Podcasts + iTunes U" at iLounge. Basically it comes down to separating AudioBooks from music and providing different sync and management options within iTunes.
Your sync differences are obvious, you get a different sync category enabling you to sync all or selected Audiobooks. One really interesting difference is that on iOS devices, media kind Audiobook lets you adjust playback speed. I haven't tried setting a music track to audiobook and then changing playback speed - might be useful for learning guitar tracks if pitch is maintained.
If an MP3 has its Media Kind set to Audiobook (in the Options tab):
then iTunes will do some different things, compared to Music type tracks:
The Remember playback position option isn't solely a property of Audiobooks, but if you're importing Audiobooks you might want to check that this is enabled.