Toilet-flushing 101 is something that should be taught in any two-year-old kid's toilet-training course. Most people don't like to see a big pot-load left by someone who was too lazy to simply push a lever. It's a turn-off to me. Except in the case of a broken toilet (like LJF pointed out), or one that "self-flushes", there's simply no excuse.

Let's not be pigs, okay?

I'm split on this. I'm sure there are some people that get a big kick out of leaving an unflushed toilet for someone to find but I am also certain that there are some people that just plain forget to flush, for whatever reason. Personally they all disgust me.

I think this is disgusting, and a sign of a dirty person. The only excuse that a person would have for leaving an unflushed toilet is one that's out of service.

Flush that thing!

That's gross.  And what's up with the urine on the seats?  How on earth do you miss when you're sitting down!?