My gripe is tipping when you shouldn't have to.
I paid $20US a person for transfers from the carnival port to the airport (2 adults and 2 children, 2 and 5 yo). When we got to the airport, the driver said "don't forget the driver" and he actually stood outside the door and waited to be tipped as people got off the bus. My husband had actually read ALL the print on the transfer and in specifically says: "All gratuities included", so we mentioned that in the bus. Needless to say, most people didn't tip. We didn't either, after all we had paid for a 2yo who sat on our lap the whole way anyway.
You never have to tip. It is something one does in appreciation of services above what is expected. Sorry, but a bus driver that takes you from one place to another and is paid for it doesn't fit into that role unless he stopped to let you buy something at a store or did something out of the ordanary to please you.