And how much does it matter that YOU are the only one who finds it hard to read? I think that's important. Just because you find it hard to read doesn't mean other people do. Or, it might be *little* hard to read for you, and you personally may not like it, but that doesn't mean everyone thinks that. And it's pretty Special Snowflake to think that someone ought to write the way YOU want them to write--and it's extra Special Snowflake to write them and tell them so.
There may be some universal (or nearly so) standards. Some that might occur to me are:
*a wall of text--like, 25 lines or so with no paragraph breaks. (but who decides whether 8 lines is too much, or 16, etc.)
*a complete lack of capitalization. And even then, I'll tell you that *I* don't have the tiniest bit of trouble reading a post in all lower case, so I wouldn't actually agree that it's universal. And if people don't cap the letter at the beginning of the sentence but DO capitalize "George," you might not *like* it, but that doesn't make it hard to read.
But other things are not necessarily universally held. Some of this stuff, it's like complaining about someone's Scottish accent. (You specifically asked about *style*, not a single specific error that might create problems in comprehension.)
lots of one-sentence paragraphs? That might be a jerky organization, but maybe that's what the person intended. And anyway, it's not like it's THAT hard to comprehend.
Text speak? Informal, yes, but not so hard to understand.
But I basically think that it isn't our place to "instruct" our fellow posters, generally. If you truly have difficulty understanding a specific post, I say you should ask for clarification in that specific thread (or ask for paragraph breaks). But I think it's inappropriate to say to someone, on a forum like this, "you should change your posting style."
And I disagree w/ MasterofSquirrels--I think most of us, when receiving constructive criticism that we didn't ask for, do not see it as "a nice way to improve [ourselves]"...but I think people do take it "as it's intended," because I think it's intended to be "you should do things the way I think you should."
I'm an editor--I fix people's grammar mistakes for a living. And I teach the people on my publication's staff by explaining what I'm changing and why. I do approach people to tell them what they're doing wrong, or to suggest ideas to them for use and improve with. It's my job; I'm specifically tasked with it. But even then, I don't go "teach" people unless there's more than one error, or unless there's something sort of big at stake.
I would never do those things anywhere outside of where I work, where I have an acknowledged and open role in correcting people.
And in fact I think that unless the spelling error or word choice is really confusing, you shouldn't mention it. And if it IS confusion, then you should ask for clarification it right in the thread.