16:9 Tablets
Due to the 16:9 display it's a little bit awkward when you're holding it landscape and really awkward when you're holding it in portrait, it kind of wants to be on a desk.
That's not really a critique specifically for the Surface though, but every non iPad tablet out there. 16:9 displays are great for TVs or desktop monitors but on tablets they are just a bad idea because, as Josh said, they are awkward no matter the orientation you hold them.