I personally find it very irritating when people don't know the difference between your and you're... Or just type both as "ur". Also, other annoying abbreviations are y, r, ik, m8, use of "u" as a word in general.
Now, I understand if someone wants to use these abbreviations while texting with an older cellphone without a touchscreen and whatnot, but doing so in general chat / forums and such is just disrespectful towards everyone else. Especially those of us, who speak English as a second or third language. In addition, poor typing just makes me (and I assume I'm not alone with this) think the person behind the keyboard is an illiterate.
It may be faster, but in general chat/forums you are rarely in such a hurry that you can't type properly... And it gives a better impression to whomever is reading it. Consider typing properly a common courtesy towards us who speak English as a second or third language, thanks!