1) Light weight: Because where I go, it goes. Heaviness is a disincentive to keep it with me.
2) Paperback-sized: Because that makes it easier to stack with the other books I'm prone to carrying around.
3) Durable: Because coffee is bound to be spilled and it is sure to be dropped and generally banged around. I'm especially careful to look for notebooks with strong and flexible bindings, and I prefer those that lie flat when open.
4) Plain: Because it is above all a working journal and I need to feel free to make a mess of it. Like other writerly types, I frequently receive truly lovely journals as gifts, but I can't use them -- filling them with my scribble just makes me feel guilty. The writing notebook has to be both canvas and dropcloth.
5) Unlined: Because it allows my creative spirit to roam blithely about the page.
6) Black: I don't know, it just feels right. Plus it hides the scuffs and stains (see #3).
Looking for a good one? I highly recommend the classic Moleskine notebooks (which is what I ended up with this weekend). Simple sketchbooks sold by art supply shops will fit the bill as well. I'm also a softie for the old black-and-white speckled composition books, though they are a little too big for my liking and the pages are lined.