I imagine that one's down to personal tastes. At first I kinda thought the same thing because it basically hides what you have open until you click on what else you're loading up, but then I realised that for the very brief moment you're loading up something else via either form of start menu, you have no need (except maybe very rarely) to be able to pay attention to the rest of your screen, you need to be looking where you're clicking to open what you're opening. After you click it, the start menu immediately goes anyway, so I don't see the disadvantage to it except for a potential degree of UI inconsistency, but I always found the start menu to be needlessly inefficient anyway, and I think Metro improves on that massively.
7 and 8 aren't without potential improvements in UI, I just find 8's to be better in some ways, worse in others. On the whole I slightly prefer 8's but not by much. That with it's improved performance and slightly cleaner look make in an overall improvement in my eyes as far as I can currently tell.