I've been using Windows 8 as my main operating system ever since the release preview was, well, released. A consistent issue - starting with earlier preview releases - was the downright abysmal quality of Metro applications, and Microsoft's own Metro applications specifically.
In general, they tend to be slow, contain artefacts, crash, hang, lose responsiveness, or just flat-out refuse to do any networking operations. Mail and several Twitter clients, for instance, sometimes just stop updating, forcing a restart. In the case of Mail, not even going through the task manager helps; only a reboot will do the trick.
Internet Explorer has issues of its own. It suffers from a 'black page' bug, where switching to IE10 from any other application will make all the pages go black until you bring up the tabs bar and reselect the page. Another incredibly annoying thing is that page loading often blocks access to the tab bar; if a page is doing some heavy loading, the tab bar will not show up when you right click, so in frustration, you click again and again - only to have the bar quickly appear and disappear several times a few seconds later as the page stops loading. UI access should never be affected by page loads - I'm hoping this isn't an architectural thing.
The media applications don't fare much better. They pull stuff from your Libraries, but they seem to have to do this every time you load them up - with larger libraries like mine, that's a process that takes not seconds, not minutes, but dozens of minutes. They also shove loads of advertising in your face (they're essentially music/video stores which, oh, happen to give access to your own files.
This is just a sample of the bugs and issues you run into, even this close to release of what's going to be a defining product in Microsoft's history. This isn't release early/release often territory; Microsoft is a publicly traded company and people are going to pay actual, real-world money for this product. Pulling a KDE 4.0 is simply not an option, but the current state of Metro's applications do not bode well.
This should be Microsoft's top priority. Metro has potential, but without quality applications, it's going to fail spectacularly. It all feels eerily similar to Windows Phone 7's usually rather abysmal third party performance, with the obvious difference that Windows Phone's first party applications are some of the best - if not the best -smartphone applications out there (WP7's mail application, for instance, eats iOS' and Android's for lunch).
The coming few months are going to be extremely crucial for Microsoft. Metro's supra-application interface elements, like the Charm bar and application switcher, perform superbly, never failing and always quick, fluid, and responsive. It remains to be seen if Microsoft can bring that same level of smoothness and performance to Metro's applications.