So, when I started thinking about this meta, my original question was going to be the same one I've had since I first saw the episode: What is Dean's deep dark secret that Bloody Mary draws on?
Except that, watching it again a couple of times this weekend, I suddenly realized I'd been asking the wrong question entirely. The question should really be: Why doesn't Dean think he has a deep dark secret?
Because it's obvious he doesn't. Hell, we've seen him stand in front of the figurative loaded gun for Sam before, so if he thought he had a secret that would appeal to Mary, he'd certainly have been out in front, calling her name as loudly as he could. So why doesn't he? It's obvious he has some kind of secret that she can draw on, or he'd have been as unaffected by her as the "innocents" in the rest of the episode. Even subscribing to the idea that, once freed from her mirror, Mary could inflict damage on anyone, you have to wonder that Dean hasn't committed some kind of hellish act that he can be rightfully guilty of, you know?
I think the fact that he doesn't feel guilty enough about anything to feel he's a suitable target speaks to the way we've already seen Dean live his life. When he comes to Sam at Stanford, he's understandably worried about their missing father, but it takes a very brief time for him to immerse himself in the hunt for the woman in white and save the day. Once it's obvious that Dad meant them to have the journal and meant for Dean do get the job done in Colorado, Dean settles down into doing what he's always done. Unlike Sam, Dean seems to have a true gift for living in the present moment. He doesn't dwell too much on the past, though previous wounds still hurt him deeply, and he doesn't worry too much about a future he can't predict. It's as if he took his father's admonishment on the night of the fire completely seriously. "Don't look back!" is, in many ways, his watchcry.
Now I'm not saying that Dean is heartless or lacks a conscience or anything of the sort. On the contrary, I think he's a good bit more responsible and moral than either John or Sam in many ways. No, what I'm saying is that Dean has lived his life trying not to be the one dragged down into the obsession. He says in the pilot that he thought the way they were raised wasn't "all that bad," and he's more than happy to hunt down any old evil thing, not just the thing that killed their mother.
I also feel that Dean's focus on the present moment is a prime reason why, here in the beginning of the first season, his character is actually written as incapable of taking the starring role. The story is about Sam not just because he's the one with the most recent hurt, but because the hurt is something he picks at daily. It's a wound he opens as often as he can. Dean just isn't built like that. And as of "Bloody Mary," we've yet to see him change that present-minded face for us.
See how I cleverly didn't mention the radical shift in the character later in the season? *snerk*