This was my biggest issue as well. It seems Metro could have been easily set vertically, but instead there was this odd choice for horizontal that just feels incorrect.
I might be wrong, but I think they're mimicking the xbox dashboard which goes horizontal since you're moving it with the right & left trigger buttons. Feels natural on xbox but i think they probably wanted a sense of familiarity and thats why its horizontal on windows? Idk. Just my assumption.
