Email etiquette for requesting a meeting with a professor who one won't do research with or take a class from?
I'm assuming that you are a graduate student.

Simply send a short, polite email expressing that you are interested in his/her research and that you would like to talk about it. If you share a coffee machine, lounge, canteen, etc, you could suggest to talk over coffee. Certainly, say who you are and what you do in the email, and indicate why you are interested.

Don't ramble on in the email. No one has time to read long emails. 

Showing interest in research and showing that you won't be wasting his/her time are main points to observe. Most researchers (unless they are ridiculously famous and super busy) would be happy to explain their work and create new avenues for future exploration.