Everyone has at least one genre that for some reason they just don't gel with. For me, that is by far the easiest thing I had to choose. Magical Girl is one which, for some reason, just doesn't work for me. I suppose that it comes from the fact that theres this amazing transformation from a perfectly normal indiviudal to someone who waves a wand around and can then embark to save the world.
I suppose that the story element behind it is the main reason as there hasn't really been any really decent anime out there which ultilises the Magical Girl genre correctly. I tend to prefer realistic characters (err...as far as realistic goes) to those which have their duel characters. At least with shows like Heroic Age, Mnemosyne and even Tsubasa Chronicles to some extent, all have female lead characters who boast magical abilities or some sort.
Yet these shows don't focus on that, rather the events around them and draw you into the story without having it thrust upon you at every available moment. As such, these shows personally, are just that much better.
I'd like to see just how many people on Anime Planet has this as least favourite genre and their favourite genre as I think that it would be a clearer divide between those who like them and those that don't; when compared with the other genres.