At this point I'm mostly concerned that they're pushing it too far towards the tablet and not leaving enough for the desktop user, despite the claims of still providing the standard windows manager. Having watched the demo videos, I'm very interested in trying this software out on a tablet: it seems like it provides a lot of the ease of use and consumption of an iPad, while still allowing the user to actually do real productive work if they wish. My hope is that this might actually provide the mythical SciFi tablet experience, where your slate seamlessly transforms from on-the-go convenience to full-on powerhouse merely by slipping it into a dock.
However, some of the stuff that is being shown is genuinely worrying from a desktop user's standpoint. One of these potential concerns is the talk that it's new Task Manager will automagically "suspend" programs in the background, much like mobile OS's. That's all fine and well on a mobile device with limited resources and battery life, less so on a powerful machine. I already find it irritating on my android phone when I switch away from a website while its loading to check an email, and when I come back I find the browser was closed in the background; if this happens on my desktop I'd be apoplectic.
The second concern is with how much they've "touchified" the UI. I've actually downloaded and installed the developer's preview into a virtual machine to give it a spin, and at the moment its fairly painful to use with a standard keyboard and mouse. That Metro grid of apps and blocks that prominently shows up on startup isn't just the touch UI that you can banish; that's actually your official start menu. If you launch the window manager, and then press the start menu button, you find yourself right back in that grid. There's no way to get a standard listing of apps, and the grid is very painful to scroll through with your mouse. Likewise, all the settings controls for the system and so on currently seem to be full-screen, touch-style applications, which again just aren't that easy to use from a desktop perspective.
I'm hopeful they can successfully merge the two UIs together so that they both can live seamlessly for the user. But I can definitely understand why people are worried at the moment.