I'll sound like an asshole (it's because I am) but grammar. It's not that hard to use the proper "there", "too" or "your". If you're not sure, fuckig google it. 

People who use coloured dialogue as an excuse to write it incorrectly. I don't care what shade of neon pink your dialogue is, you can't have more than one character speaking in the same paragraph. 

Capitalization. Oh my Christ it isn't that hard. I've come across several people who capitalize ever goddamn noun and even verbs and adjectives. You capitalize the beginning of a sentence, proper nouns (AKA names) and acronyms. [i]Thats it.[/i]

People who insist on writing everything (IC AND OOC) in italics or coloured or [i]both[/i]. You don't have to be a special ducking snowflake. Write normally. I can't read that shit. Even if you [i]must[/i] italicize your OOC chat for some goddamn reason, [i]dont italicize everything IC.[/i] Italics are for emphasis and thought. If you italicize everything, you're both confusing the audience and looking like a tool. 

Whew, now I feel better.