I think people are complaining because xkcd is a high-quality site that everyone knows about (and likely reads already). In other words, just about every comic on the site could be submitted here, but most of us refrain from doing that.
Then again, the same thing could be said for PG's essays, but I guess in that case, there's always some kind of discussion to go along with them, whereas there's not much to say about a comic.
I find that many blogs in my newsreader I still read mostly through YC news submissions. I don't read all the blogs in my reader on a regular basis, and somehow I count on YC news to notify me if something especially interesting appears in them.
I wonder if some of the people complaining about notable items appearing on the front page of Reddit are some of the same people complaining about non-notable items appearing on obscure pages within Wikipedia.
