1. People who can't back up their points in debates/arguments.
2. People who make their opinions seem superior to yours.
3. People that talk WAY too much.
4. Unhealthy people, stupid people.
5. Any sort of discrimination without purpose. Hate without a reason is unacceptable.
Lots of other stuff, but that's all I can think of off the top of my head.
It's rare that something frustrates me immensely - but:
People who simply will not listen. Who know they are right, and will not look at other sides of the issue at hand.
Normally I can choose not to talk to the person, but on occasion I am stuck in a conversation via university, or the fact that it's someone I care about. I've started to simply change the subject, because it frustrates me fiercely.