You aren't writing a book; you're talking to someone. IMing in completely perfect sentences does give an air of distance.
There's something to be said about taking the written word at face value and trying not to read into it much. I suspect the latter is because, without verbal cues or body language present, the human brain requires addition input to read the emotional state of the writer. Without that, it examines things that are probably incidental and meaningless but certainly not without consequence.
We at HN are probably not the most emotionally connected anyways, so there's going to be a huge gap between the average HN experience/opinion on this topic vs. the general population.
This is probably quite true, particularly for those of us who are of the INTx MBTIs.
That said, my sarcasm was an attempt to illustrate the silly nature of this entire debate. Although I do agree with the OP (insofar as I can't help myself from feeling lax capitalization and punctuation is lazy), I also can't help but feel that getting worked up over punctuation is rather stupid.
We're probably in agreement to that end, though.
First impressions are important to humans. "y ru not here" will be judged very differently from "Just wanted to make sure you're still able to come."