"Great minds discuss ideas, average minds discuss events, small minds discuss people."
Football/NBA - I love football just as much as most other Brits, I like to support my home town by keeping up to date on where they are in the league and very occasionally going to see football matches. Boys, however, seen to find it necessary to watch every single match they can manage to catch on the TV or the internet no matter what team is playing. It's very irritating. The same applies to the American students I'm friends with and their obsession with NBA. Not to mention they have to stay up until 5am every night for a week to watch the playoffs/finals/whatever-they-are!
Cleanliness - Most girls are under the general impression that boys are smelly and dirty. After living in a house with 5 boys for the best part of the year I can confirm that, yes, boys are very smelly and most of them haven't been taught how to pick up a hoover or a dish cloth (I am lucky though to have a very house trained boyfriend!). Living with 4 other girls for a year also taught me that girls too can often be extremely dirty, sometimes more so than boys. Ew.
Play-fighting - I have a sister and two male cousins that I hardly see, so my impression was that boys stopped play fighting in their early teenage years. This is not true. I have so far witnessed boys up to the age of 23 play fighting and it mostly doesn't turn out how they imagined when they return nursing broken bones and bruises!