People who've forgotten their headphones but still insist on listening to music at an obscene volume. If I forget mine, I don't assume that everyone else on my train or bus will be happy to listen to my mix of Half Man Half Biscuit and Fugazi, so I do without... for some reason if someone's "bag" is whatever rubbish is big at the time, they assume that everyone else wants to hear it too. You can get a fresh pair of headphones for the cost of a coffee so there's no excuse.
It's not just the "youth" either: there was a bloke the other day, middle-aged, well-dressed and carrying a leather document wallet, regaling the entire carriage with his personal DJ set. At first it wasn't too bad with a mix of REM, early U2 and even The Smiths... but next up was that awful "You're Beautiful" by James "Rhyming Slang" Blunt!