I pick my battles, but generally have no fear in being polite and asking people to stop being idiots.
Man smoking on the bus? I would have spoken to the driver and asked for a copy of the drivers details (they can print a blank ticket with this info on it) and complain to the transport company or authority if First/Veolia/TM/Stagecoach dont care.
Ive asked a teen to stop playing his music loudly. I went up to him, with my phone in hand, and asked if he cared to listen to what I was. He said no and I told him, well I dont care to hear what you're playing either. He got all sheepish, coward into his hooded sweatshirt and the music went down. The driver thanked me as I left the bus. If he was with a few more of his mates and not by himself, I probably would have stayed quiet.
The head phone thing is annoying. I wish people would realise the bog standard headphones you get with your mobile or mp3 player only fit a select few in this world. The majority of us have different sized ear canals and because of ill fitting headphones, the music leaks and is shared with others. A cheapo £25 Sennheisers with adjustable ear pieces will work for most people. I leave the headphone thing down to shear laziness.