It's a little thing, but one of my pet peeves is the seat-yourself-ers. Of course, that annoys everybody in a restaurant with a host. But what really makes me enraged is people who tell the host they're going to sit at the "bar" .... and then happily plop their asses down at a table in the vicinity of the bar.
It pisses off the hosts, because they get yelled at by the manager if people are spotted without menus. It potentially pisses off their server, if they've now been double or triple sat. It pisses off other servers, who feel they're being skipped in the rotation.
Most annoying of all are people who do it even after being told, many times, that this is not a seat yourself establishment. We have a couple who come in every Thursday. Everyone's seen them there with other significant others, so we think they're having an affair. They walk in, sit themselves at a table near the bar, sit so close they could crawl down each others' throats, and stay for up to two hours .... and leave a two dollar tip.
We've told them we're not a seat yourself place. We've told them the bar is not a free-for-all. If there's some reason they want that particular table, they can ask for it instead of ignoring the host completely--walking right a person greeting them--and dropping their rude asses down where they want.