Raging atheists who stuff their atheism down my throat are as annoying as religious people who do it. I might not believe in a God, but if I can survive without forcing this point on others, so can you.
People who define themselves by their sexuality are irritating. My sexuality is a part of me, for sure, but it is not all that I am. I don't feel the need to express myself by highlighting my preference to men as my entire being, neither should you. We are made up of many facets. We are made up of all our experiences, all our thoughts, all our ideas and beliefs and personal preferences. Let that be who you show to me, not just one tiny part of you.
Marijuana should be legalised. Perhaps if it is regulated and distributed like any other legal drug, we mightn't have such a negative attitude to it.
Binge drinking cultures are a negative thing. Drinking is fine. Growing up with the belief that you need to get wasted as soon as possible and as often as possible is harmful to yourself and the the others who see you like that. Experience drink sensibly. Learn to love and savour some of the greatest alcohols out there.
People who hate smokers - why? Let them smoke. As long as they aren't - quite literally - in your face about it, then it's fine. Non-smoker here, and I don't get up in arms when someone sparks up until they blow it into my eyes. Then it's on.
Highly authoritarian people. Sure, some level of government intervention can be fine, and is probably needed in the world we live in, but there's such a thing as taking it too far.
I'm still a Liberal Democrat (UK politics yaaaaay

). While in an ideal world I'd like to believe in some form of anarcho-communism, I'm realistic. Clegg took the offer to get into power because if he didn't then he'd have been an ineffectual politician - why would you support a politician who didn't want to get into power? However, he has reneged on some of his promises, even ones that were part of the Coalition agreement. While this has weakened my trust in him, I still think that currently the Lib Dems are better than (or could be better than, if they ever properly got into power) New Labour, and certainly better than the Tories.
Finally, this might not sound controversial, but I think perhaps it is in our world. So, here it is:
Be Nice.
It's not too hard an ideal to follow, is it? I know I'm grumbling in this thread, but I don't go out of my way to hate things. I have a positive view of human nature; I reckon being nice and getting along is a great thing. Unfortunately, plenty of people seem to disagree
EDIT: Forgot my most controversial opinion: Every generation of
Pokemon is great.