Women in a public restrooms who complain about the smell. It's a freaking room full of toilets, what did you expect it to smell like, raindrops and lavender?
For that matter, people that pee all over the toilet seat in their attempt to hover over it. For the love of Pete, "If you sprinkle when you tinkle, please be sweet and WIPE THE SEAT!"
People that do not yield their seats on buses/trains to the elderly, disabled, people with small children. Have you no home training?
Speaking of public transit, is it really necessary that the whole bus hear your music blaring or you cackling at the top of your lungs on the phone? Invest in some good headphones, lower the volume a few decibels and try not to inform the entire bus of your baby daddy/mama drama.
Slow walkers! I know my chunky butt moves slowly, but come on people! When in a mall, on a major city street or in highly populated area, it is customary to walk on the left and allow people to pass you. You shouldn't just stop dead in the middle of a walkway to talk on the phone or tie your child's shoe. 'Pull over' to the side to let the people behind you pass.
Saying to a pregnant woman, "You still didn't have that baby yet?" No jackhole, she didn't. If she had she wouldn't be there listening to your inane questions. She'd be home with her baby. Sigh.
These are really all lack of tact and/or courtesy. Do parents not teach manners anymore? There are dozens more I'm forgetting, so please feel free to add your own hates.