Yeah, I am not sure what the heartburn is about the Metro UI. The start screen is pretty much what was in the old start menu except laid out as tiles. Instead of being simple menu items, they can now show previews in the form of live tiles. You can get up-to-date "glances" at information that indicate whether or not a certain application needs your attention. Do you have a new email? No need to launch the mail program to find out. Want more, ok launch it. What's the current weather? It's shown on the tile. Want the full forecast -- click on it. It's very much like desktop gadgets integrated with icons but it's now a primary function of the operating system not some add-on.
Applications written for Metro, will show as if they're integrated into the start screen. Applications not written for the Metro UI, launch from the start screen but run on the Windows desktop. There is a desktop tile to access it directly.
It takes about 15 minutes to get used to the navigation. It works fine with the mouse. I don't think the scrollbar is very elegant for the side-to-side navigation paradigm but we'll see if that's the final UI gesture they have planned.
WMC is there. Right-click on the start screen, select "All Apps", right click on "Windows Media Center", select "Pin to Start" and it's there on the start screen.
I think what is awkward is the going back and forth between the UI of the start screen and the UI of the desktop. It'll be interesting to see what kind of universe this creates. I can see some applications have just a Metro version, some have both and some having just a desktop version. That means learning two kinds of interaction if your applications are in both spaces -- some in one and some in the other. This reminds me of the days when not everything could be controlled via mouse. It'll take a while for the Metro UI and the desktop UI to become one.