For instance, I always observe the rules of accentuation very strictly in written Spanish, including online chat, whereas most of my native Spanish-speaking friends barely use accents at all -- even though they have accents on their keyboards and I don't! One of the first things native speakers always compliment me on is my use of accent marks. And what really pisses me off is improper spelling and accentuation in print.
I used to follow Spanish accentuation rules strictly when chatting, but I would sometimes get negative feedback from people I didn't know, they'd usually think I was pretentious, stuck-up, boring, etc. just because I wrote correctly. Now what I do is watch how the other person writes. If they write well, I write well. If they sound stupid and have bad ortography, I'll instantly 'dumb down' my writing, including: omitting accentuation marks, using abbreviations, conjugating some verbs wrongly, avoiding uncommon vocabulary, losing respect for verb agreement in the subjunctive mood...
I also do this in real life (everything I mentioned except the first two, of course). Sadly, writing and speaking well don't make you friends.