Every single time I try to do something like ask a question or view tags here I move toward the place that menu item is on every single other stackexchange site. The exact position differs slightly, but its never drastically different from where you would expect. I don't see why this sight should break that trend, making switching between sites very easy. Making the UI on all the sites similar would help solve the problem of being able to identify a stackexchange site.
(to the designer: sorry for the harsh words, its just easier for me to write that way. You are doing an awesome job on all of the new sites! The designs are perfectly fitting. Just trying to see if anyone else here is of the same opinion about the horizontal vs vertical menu change. :)