Why I don't like it: It seems like the same people who would write crappy overwrought diaries in RL use the same approach and material in their poorly veiled fiction. Also, if we are talking specifically about Sam, I see too many depictions of bad writing that just aren't plausible from big-brained, Stanford-bound Sam, even when he is younger.
I'm willing to make an exception for: Pages of a Life
Why the pass?: OK, I still found Sam's writing a little weak, but it had a flavor of having been written in stolen moments, so it was doable. What I liked was that although the diary gave insight into Sam, and provided food for thought to the character who read it, it didn't magically solve all the problems between the two characters, a quick-fix which seems to be another weakness of diary-centric stories.
Why I don't like it: It is a huge challenge to convey something in a limited number of words. Many writers take on the challenge, but I think few really rise to it.
I'm willing to make an exception for: A Little Bit of Lucifer by locknkey. 146 words.
Why the pass?: IMHO, I think the author met the challenge. Assuming the audience was already familiar with the setting and the characters, there was a lot that didn't need to be said, and the author went directly to the well-done little story.
Why I don't like it: Well, there is au and there is AU. Most fanfiction is by its nature a little au. I have trouble with the kind where the characters get dropped into ancient Rome, or have careers as African safari guides, or something totally off the wall. Sam and Dean are who they are because of particular events in their lives, and if too many of those are taken away, I don't see how they get to become Sam and Dean.
I'm willing to make an exception for :Purified by Innocence (set in medieval times)
Why the pass?: Gah, I don't know. They're not even brothers here, which should be another inviolate aspect of who they are. The story is just that good, and provides a satisfactory alternate backstory for why the characters hunt evil.
So, what are your exceptions, and why? Bonus points if you can convince me to get on board with: sis!fic, genderswap, Sam/Ruby, Sam/Gabriel, or mpreg, none of which float my boat enough for me to call a single story a favorite.