People on the bus who for some fucking reason don't understand that headphones on plus book in hands equals fuck off I don't want to talk to you. Chatty motherfuckers on the bus in general irritate me, but those persistent once make me wanna choke a bitch.
People who get on the bus reeking of body odor and/or shitty weed. Persistent nasty and irritating smells when you're stuck in a relatively small metal box on wheels with a bunch of other people are pretty fucking awful.
Panhandlers who lurk at bus stops and ask for change under the guise of needing to buy a bus ticket to get home. I usually get little packs of pre-purchased but unmarked tickets to avoid having to fumble with money every time I get on the bus, and every time I've felt generous enough to offer someone one of those they're always super obvious about being disappointed that I didn't give them money, and a couple times they've just walked away without taking it. One time I gave a dude a ticket and then watched him go over to the other end of the bus stop and try to sell it to someone else, and ever since then I've been tricky with the beggars: I start by asking how much they still need while reaching into my pocket, they usually say just 50 cents or something near there, then I pull out my blank tickets and offer to trade them one for the two dollars or whatever they already have ($2.50 for a bus ticket, so I split the difference on what they asked for) because that'll be effectively the same as me giving them the rest of the money they need, and nobody has taken me up on my generous offer yet. Seriously, I have more respect for the dudes who just come by and ask for spare change, because at least they're not trying to bullshit me out of my money by making a pathos plea of "poor me I am stranded please help." Bastards.
I ride the bus a minimum of 10 times a week now, to and from work 5 days a week, 35-45 minute trip each way depending on traffic conditions, not counting time waiting at the bus stop. That ought to be enough to explain the above gear grinding complaints.