On forums I used to put high priority on a semi-formal style because I was hecka arrogant and of the opinion that perfect grammar and that makes you better than others idk. But screw that, man. I've ended up very casual- I pretty much type how I speak, plus some shorthand/abbreviations. I think tumblr and extensively IMing has helped with that transition. Capitalization ain't so important to me anymore, a wide array of things can be used as punctuation for tonal communication, smilies are rad, and various slang makes its way into what I'm saying. That's all probably helped by the fact that my posts are stream-of-consciousness. I mean this is a form of communication in a casual social setting so whatever. Don't be judging people on how they personally prefer to communicate- as long as you understand what they're getting at, why bother with critiquing their style? I'm all about clarity tbh. I try to have my message and tone come across well and try to understand how other people's styles reflect their tone. That's an important thing in a text-based world.
So really, the only posting styles that irk me are things I can't read at first glance for reasons of color, font, that Homestuck/l33tsp34k nonsense, incomprehensible word scramble, whatever. If I have to make a real effort to find out what someone's trying to say to me it's annoying. Slang's fine, being silly is fine, but don't actively communicate with me in a way that's hard to understand pls.