1. "Oh, it must be global warming". No. No it mustn't. Just because it's an unusually warm day in February is in no way evidence for global warming. Nor is there any particular reason to believe that it wouldn't have been that way without global warming. The existence of global warming is now demonstrated beyond reasonable doubt (as I understand it; IANA climatologist) but doing so took an awful lot of debate between an awful lot of smart people looking at an awful lot of data. It is not something that has anything to do with "hey, it's sunny today".
2. *shrug*. It may just be me but I always read this as the world's most obnoxious and passive-aggressive emote. It always comes across to me as shorthand for "well, I think that you're completely wrong, but I honestly don't give enough of a damn about you to try to argue". That may or may not be how it's intended, but that's always how I read it.
3. Those bloody annoying things that people have on their userinfo pages saying how many days/weeks/month it has been/will be since/until some sort of event. Honestly what's wrong with just saying "such a thing happened on $date"?
4. Wattage. Tonnage. Yardage. Ampage. Anythingelseage. The words are "power", "mass" (or "weight"), "distance" and "current". I have never understood why people insist on doing this, and I hope I never will.
5. People I don't like showing up in completely unexpected places. OK, so this is actually a rational dislike, but it still annoys me.