I used to be a generic reader, one of those people who read cereal boxes and pamphlets in the doctor's office about male urinary tract infections. I'm still voracious, but I find that the more I write for myself, the more critical I am of what I read. I can no longer finish a book if the author doesn't create characters I respect, and by that I mean characters who ring true to me. Even with some authors I admire greatly, I find myself asking, "Oh, come now, would a straight-laced, uptight Victorian really lose it and scream at her husband in front of a guest?"
Likewise, a mystery must be a true mystery, not something manufactured for the sake of a book. Often when the story comes to the end I find myself concluding that the whole thing would have been better served if the protagonist had gone to the police, told them what he/she knew, and stepped out of the way to allow professionals to do their job. Of course I know that can't happen if a story is to unfold a certain way, but the author had better prepare me with reasons, not just let the protag go off on his own because he's too cool to do otherwise.
Age and experience often interfere with a story as well. The book I'm reading right now is historical, and I know enough of the period to disagree with the author's take on the situation. This doesn't bother me too much as long as the groundwork is laid well, but I still hear little arguments cropping up in my head: "Old Henry Eight was not quite the fool that this author purports him to be."
It's a combination of years of reading, of teaching communication, of practice writing on my own, and of looking at the elements and making sure they are all served as well as possible. A heightened sense of what comprises good storytelling makes it difficult to enjoy just any old book these days. Of course, when a great book comes along, one where the author gets everything right, I appreciate it more than I ever did as a twenty-something reader.
The killer is that those really well-done books make me admit that as an author, I'm not at the point I want to be yet. Not only do I see the faults of other writers; I see my own and think, "You shouldn't be reading anyway. You should be working at becoming a better storyteller."