I really hate it when I walk into the men's bathroom and find all (or most) of the urinals filled with yellow water. It's just gross. And sometimes, if it's been a while, you walk into the bathroom and get assaulted by the odour of stale pee left in the urinal. Eewww.
What's even worse is when I'm doing my business, and the other guy finishes his and leaves without so much as a flush or attempted flush. Sometimes he'll give a quick whack to the handle, which is completely ineffective. And it's not like they don't know it didn't flush properly. Urinals make distinctive noises when they're flushed successfully. A quick yank on the handle and a little bit of water dribbling down does not a flush make, and definitely does not make that distinctive sound.
Guys, I mean really, is it that difficult to push the lever or button to flush after you're finished? Let the water flow and run your own waste water away. Leave a nice mostly clean urinal for the next guy. Do you leave your toilet at home unflushed after doing your business? Are you trying to mark your territory and lay claim to the bathroom? Is the next guy supposed to walk in, smell stale pee and think "Oh, this one belongs to someone else...I'd better find another one"?
I mean, I know guys are supposed to be gross and disgusting creatures, but really. Flush, please! It's not that hard!