Public toilets (ladies only, I don't have any experience of the gents.)
1. People bringing food/drink in with them, it's wrong.
2. Ladies bringing their sons in with them who are way old enough to be using the gents, who unnecessarily add to the queues in the ladies.
3. People who do not wash their hands.
4. Parents who refuse to parent - the toilet facilities at a venue I visited at the weekend consisted of 1 ladies loo. Immediately before me were a lady and her daughter (probably about 8 years old). As they passed me on their way out of the toilet the daughter said to her mum "I don't want to wash my hands" and the mum and child just walked out. I'm sorry, but if my sons were daft enough to say that to me they'd get short shrift. Toilets - home, public or otherwise - you wash your hands, end of discussion.