It's not even just the fusion of desktop and mobile in principle, but the failure to execute that concept. My mom took to her iPad like a fish to water, and quickly learned to send e-mails, surf the web, receive and send photos, etc. Meanwhile, I had a Surface RT for a bit, and while Metro was slick, I found myself constantly being switched to the desktop. Want to open a word document as an attachment? You have to flip to a completely different UI parallel and barely interoperable UI to do it. Even a lot of the basic settings were duplicated incompletely between the two worlds (I haven't tried Windows 8.1 myself, so I don't know). It's simply an unacceptable design feature for a consumer-level product.
I don't think the concept is flawed. I actually think Metro is the "right answer" for your typical user, with a hidden file system and tiled window management, even on the desktop. However for this to be plausible, you need to be able to ship without the desktop, which means Office needs to be Metro-friendly. The stop-gap of shipping with both Metro and the traditional desktop is a fatal execution flaw in what's otherwise a reasonably sound design.
A better, technically possible, solution would be to open Win32 apps as full screen apps within Metro, and allow the usual tiling and app switching. This is basically how OS X works, and it's very effective and also quite efficient if using a touchpad that allows gestures.
I don't think the concept is flawed. I actually think Metro is the "right answer" for your typical user, with a hidden file system and tiled window management, even on the desktop. However for this to be plausible, you need to be able to ship without the desktop, which means Office needs to be Metro-friendly. The stop-gap of shipping with both Metro and the traditional desktop is a fatal execution flaw in what's otherwise a reasonably sound design.
A better, technically possible, solution would be to open Win32 apps as full screen apps within Metro, and allow the usual tiling and app switching. This is basically how OS X works, and it's very effective and also quite efficient if using a touchpad that allows gestures.