To summarize:
Egregious spelling and/or grammatical errors, to the point where the process of getting at the content of your message could best be described as "deciphering" rather than "reading" - that's bad.
Wall-o-text, also bad. Particularly if combined with the above, but even if the writing is otherwise fine but paragraphs are enormous.
Nitpicking occasional or forgivable errors, *also* bad. It's making too big a deal about nothing and giving the impression that it's not even worth *trying*, because you'll hear about it if you're anything less than perfect.
Using abbreviations - situational. Stuff like "lol" and "btw" are easily recognized abbreviations of significantly longer phrases (and lol in particular is never used unabbreviated anyway) and aren't really a problem.
Stuff like "u" and "r" and "2" is abbreviating a whole one or two letters out of a word, and while it may be viable in a game or IM conversation or anything where the *speed* at which you type stuff is important ... it's just plain pointless to do in a longer thing (like a message board post) that you're taking the time to *compose*.
At that point you're sacrificing readability for, really, absolutely no purpose (congratulations, you have annoyed/confused your reader to save yourself approximately 0.2 seconds and one keystroke, you have an *excellent* grasp of worthwhile tradeoffs). But again, there's gray area here, and people will have differing opinions on what's reasonable and what's not.
Pointing out the particularly blatant problems (*not* nitpicking a few things in otherwise readable posts) - that's good. Politeness is a good place to start but if they don't bother listening (they ignore you entirely, or they spout off the good old "this isn't school" "this isn't English class" "this isn't a thesis" "it's only the internet" arguments, or whatever), then there's no reason to continue showing them the respect of politeness when they don't show you respect in return.
It's safe to assume that anyone who's been continually chastized for being exceptionally unreadable on these forums, and is *still* exceptionally unreadable (i.e. their posts must be "deciphered", not "read", as I said above) is long since past the point where it's worth bothering to be polite to them on the subject.
Summary over, my two cents:
Of course, common sense says that if they've heard about it for this long and still haven't improved, they ain't gonna improve in future and it's not worth giving them the power to put you into an aggravated mood on the subject. Therefore, I reiterate the best advice for the chronically unreadable posters (such as Blazn, Yusuke, Hellstorm) - if they make a typically unreadable post, simply pretend they didn't post anything and go on with the topic without deciphering or replying to their posts. If they make something readable, feel free to reply to it.
Giving someone your full attention (and indeed, extra attention for the "deciphering" part) even when they couldn't be bothered to lift a finger to write readably, means they have no motivation whatsoever to do so. However, if it means that they effectively stop getting paid attention to, that loss of attention would be a far more powerful motivator than any criticism. It's like dealing with children who act out, after all - they want attention and don't care if it's good or bad, and the best way to get them to behave is to ignore them (or only give them the most passing of acknowledgment) unless they do something deserving of *positive* attention.
