"Epic win"
"Epic fail"
"Owned"
"Pwned"
Anything 1337
"That's like, your opinion, man"
"The cake is a lie"
"Atheists are a religion"
"U mad" (this one only when it's used out of context. I know it's a trolling mechanism, so when used properly, it's ok)
"Get a life"
you're instead of your and viceversa
"Bro"
"Imma let you finish"
"Up in this *****"
"Gem"
"Oh, it was pretty horrible, I did really bad" - proceeds to show a 100% score or a perfectly executed masterpiece drawing/sculpture/whatever. tl;dr fake modesty
"I don't know anything about this subject, but here's my opinion"
"I don't want to be racist/a jerk/sexist" - proceeds to be racist/a jerk/sexist
Anything in japanese. "Kawai", "desu", "ugu", and the infamous "~" that they had to google what key was it binded to in order to type it. Damn weeaboos
Emoticons. ALMOST ALL OF THEM. "xD", "<3", "T.T", etc. The only acceptable ones are "
", "
", ":|", and maybe "^.^" if not too overused.
And a lot lot more than I can't remember now...