Well... about performance, I don't see that as a true argument, I have W8 RP on a virtual machine and it goes pretty well, is fast even with 1 GB of RAM (talking about x86 build)...
I guess the true fail is the interface, in Vista, Aero was just an eye candy, in 7 Aero has functions that really makes you productive (Aero peek, Aero snap...) and allows you to work with piles of windows at the same time and never get lost in that...
Windows 8 is several steps backwards from that, Metro is annoying as you got to do more steps to do the same, now you got to click on the desktop tile if you want to go there... even worse, if you use a metro app and want to do something in Desktop, you need to "entirely leave" metro enviornment in favor to do "real" stuff.
To go to all programs "screen", you need to do a right click... in start menu you only point that option and it opens by itself, everything listed and ordered properly in a non big list of programs... Start Page has only one advantage though, the way of ordeing items, now they are ordered hoizontally allowing to pin more used stuff in the main part, but at the cost of more wasted time clicking around the interface... and also another bad point (talking about customization) is that you can change your wallpaper in desktop and the images shown in the lock screen, but you cannot do that in the Metro Start Screen... ?????????
The most important thing here for me, is work load efficiency, Metro is improductive if you work with lots of windows, you got no close button on metro apps (C'mon MS... just a simple damn button, it won't cover the whole screen... can also be used by a finger!!!!! ¬¬) and now right click is the only way to do things (open new tabs in Metro IE10) if you want to avoid using a keyboard and the win + shortcuts... is just slow and sluggish if you are just used to just click a lil' icon and get instantly a new tab...
Maybe Tablet and SmartPhone users will be pleased with this, but thinking a while about the PC users, this will be a nightmare not only for people like us that, like it or not will be used to the interface (that's what we do), but for those who only use a PC for basic stuff and learned a more straightfoward way of doing things...
They need to separate interfaces, Metro (or whatever they want to call this goddamn thing now XD) and classic desktop with Aero (It already has this by fortune!!!!!) for PC users, this will solve the main concern of this OS, giving the impression that is a toy instead of a tool...
Just my two cents though