We can't eat on buses (though the trouble is that some of the people who would are not the kind of people you'd want to risk saying anything to - with most antisocial behaviour really).
The worst on our buses seems to be the free papers which just seem to end up all over the place by the end of the day. Cans (altough drinking banned) or bottles is another one, often with whatever was left in them leaking all over as they roll about the bus - especially stale alcohol.
Similarly with smoking, especially dope and in the winter with all the windows shut. Don't see it so often now though since CCTV installation and often ticket inspectors on buses are now accompanied by the police. I've seen a drugs raid with police and sniffer dogs on the tram too. Attacks on buses happen though, especially in certain areas (one just the other day) and there are areas the buses sometimes refuse to go through after a certain time for periods.
Eating/drinking is ok on the tram - I'll often take a coffee on.
I've seen some nasty stuff on buses over the years - the worst being a gang of guys laying into each other with metal bars and having to ask to get off the bus (worried they'd fall on me as right behind me). No police called, they bus driver just pulled in, let us off (when asked!) and some of the guys legged it a bit after and we were let back on the bus (full of blood). Also seen an altercation end up in two guys going from the bus door back through a glass shop door, a gang threatening to find a man's young daughter (who was with him) and rape her as he told them to mind their language, been on a tram under attack between rival football fans...
I so need to learn to drive!