I make an effort to use proper grammar no matter how I'm communicating. That doesn't mean I'm going to diligently edit my posts mind you; instead, it simply means I will attempt to do it properly the first time around. I make plenty of mistakes of course, but I at least I know that, if I am misunderstood, it is probably because I failed to clearly convey my ideas.
In games and in texts, I don't mind when people leverage the various methods for abbreviating language. Often such things are written by someone in a hurry or with a strict character limit. It does irritate me (just a little bit) when someone does the same thing in an e-mail or on a forum where you have all the time in the world to spell out a word. The reason it irritates me is simple enough: it tells me that the writer didn't care enough to take the time to write their message properly. It irritates be because such actions are an insult (however minor) to anyone who participated in the conversation up to that point.