Mpreg is one subgenre I just don't really "get." I feel like I may have stumbled across that in HP fanfic first, too, before I ever saw it in BSB, but I may be remembering wrong. There's a lot of weird stuff on fanfiction.net LOL.
I don't think there's anything I absolutely won't read, though I find things like mpreg and slash hard to take seriously. The suspension of disbelief factor prevents me from fully enjoying those storylines.
I always used to think I would never be able to get into an AU, but I've found that when an AU is well-written and the Boys are portrayed as the same guys we love who just happen to be living different lives, I can buy into it. "Between the Lines" is the story that changed my mind about AU, though it took me at least two tries to really give that one a chance.
I'm also not a big fan of sci-fi or fantasy, but every once in awhile I come across one that is well-written and interesting without being too "out there" for me to enjoy. "On Nightingale Hill" comes to mind.
I will read romance if there is a bigger, more interesting storyline to accompany it, or if the characters are so compelling that I actually don't mind just reading about their relationship - a rarity for me, but it has been known to happen. My favorite fanfic is "Cover Me With Dreams," which is Romance with a capital R, but it has the right combination of good writing, dynamic characters, subplots, and plenty of drama to keep me interested. That one changed the way I viewed romance.
I also don't usually read Kevin or Howie-centered stories, but I will if they're paired with Brian or Nick (in a non-slashy way) or maybe if it's a medical drama.