Capitalizing the first letter of every word, or every other letter. Especially irritating when you're trying to roleplay and one of the participants will not stop typing like that.
"Definately" as a typo for "definitely" doesn't bother me nearly as much as when people use "defiantly" instead. My brain can skip over "definately" as an obvious typo, but "defiantly" really throws me off since it's an actual word that could be grammatically correct (although nonsensical) in many of the sentences where it's misused. Even after I've concluded that it was supposed to be "definitely," my brain still can't help trying to interpret it as written. Same problem with "discrete" for "discreet," "diffuse"/"defuse," etc.
Also, I hate it when people use non-standard forms of emphasis for no good reason. We have plenty of good tools for emphasizing words. On computers, we have italics, boldface, and underlining to emphasize or call attention to text. If only plain text is available, various substitutes have become common, such as bracketing the word in *asterisks*, surrounding it with _underscores_ to evoke underlining, or the (sparing) use of ALL CAPS. So why do people with all
of these options available instead choose to emphasize their text with Random Non-Standard Capitalization, replacing spaces with periods between every.word.in.the.emphasized.phrase*, etc.?
*I have no problem with the (sparing) stylistic use of sentence fragments. That. Is. Different. One is taking artistic license with the rules, the other is simply ignoring the rules willy-nilly.