Typically I don't expect anyone to bring anything. The usual call that goes out is "I have x, y, and z - if you want something I don't have, feel free to bring it", and if they don't bring anything, I assume they're fine with what I've got. So I usually don't mind if people don't bring things.

I usually bring something to parties either if I know there's a specific theme (thus I know what to bring), or if I've had time to stop and get something or make something. But sometimes, time runs out and I assume people understand that, so I don't usually skip something if I'd have to go empty handed.