When WinPhone first came out with the Metro UI I was a fan - there's a visual simplicity to it that's very appealing. After you use it for a while though the weaknesses become pretty glaring and hard to accept. It is often very hard to tell what UI elements are interactive and what are purely informational because they are so plain. There's no way to visually discern a non-interactive icon vs. an icon that is also a button.
The lack of shading and UI chrome also means that UIs frequently become jumbled. Sections of UI blur together where on any other platform they would've been separated by a visual line, shading, or something else.
The simplicity in this case has gone too far.
It's also very true that many of the first-party apps have ludicrously low information density, almost as if they expect these devices to be toys. This is in stark contrast to MS's stated goal of shipping something that is more serious, more productive than iPads and Android tablets, which up until now have been seen as leisure devices.
People often accuse Apple of taking style over substance, but Win8 IMO is a far, far more egregious violator.
There's another big issue: the first party apps suffer from some pretty serious performance problems. It doesn't bode well for your platform when your own internal teams can't ship best of breed apps. The People app, for example, takes literally 6 seconds to load your recent notifications on a Surface RT - all the while without displaying any loading indicator. You literally tap the button, wait, figure it's broken, and just as you're about to move on it pops into existence - and of course the performance is so poor that it just magically appears on screen without transition.
The entire OS is littered with sloppiness of this variety - as well as apps where touchability has clearly never been comprehensively addressed. You will move from places with gloriously comfortable touch targets (like the home screen) to apps that have 9pt text links you're expected to hit.
The "search" charm is also poorly thought out. Just take a look at Amazon, eBay, iTunes, and what have yous that have substantial search functionality - Windows expects everyone to cram their search needs into a single freeform text input. In fact, the eBay app on Win8 builds its own search page. Surprise, search is complex, context dependent, and not all apps can pigeon-hole it into your paradigm. Oops.
[edit] Extra rant: I was able to get the Windows Store app completely stuck today on the Surface. I visited an app's detail page, and tapped the Back button to get back to the search results. Nope. Back button would visually indicate interaction but do nothing. Waited, nope. Sloppy bug.
So here's where it gets good. On any other platform (and in old Windows land) I could just go kill it. Except I have no idea how to go about quitting an app on Windows 8. Apple at least has the courtesy of allowing you to kill an app very quickly - if someone knows how to do it in Win8 I'd love to know, because clearly their own first-party apps are not good enough to be trusted to take care of themselves.