Tablet-oriented mail client
The Windows 8 Metro Mail app is very pretty -- and very underpowered. Want to set rules for how it should handle incoming messages, such as automatically routing some messages from certain people to certain folders? It can't do that. Want to use threaded messaging? It can't do that, either. There's plenty of other things it can't do as well.
It measures up quite well to other tablet mail clients. But it's anemic compared to a fully functional PC mail client.
Less useful Desktop
On Windows 8, the Desktop is far less useful than it was on Windows 7 or Windows Vista. There's no start button, and so it's much harder to find, organize, and run apps. The Launch Bar is gone, making it harder still to launch frequently used applications.
For tablets, none of this matters. For PCs, it does.
Can't boot into the Desktop
Tablets are built for consuming content; PCs are primarily built for creating content. So when you use Windows 8 on a PC, you'll frequently want to get right to the Desktop, so you can use productivity apps such as Microsoft Office. Like it or not, in Windows 8 you can't boot into the Desktop. You'll first have to boot into Metro, then run the Desktop as an app.
Aero gets killed
The Aero interface, first introduced in Windows Vista, is being killed in Windows 8. In its place will be a plain, flat-looking interface.
Not everyone is a fan of Aero, but it's always been nice to have the option of running it. Microsoft claims it's killing Aero because it looks "dated and cheesy." More likely, though, it's killing it because the Aero effects suck up battery life on tablets.
This doesn't mean that Windows 8 has nothing to offer for traditional PCs. There's a lot to like in Windows 8, especially some of the newly released Metro apps for sports, news, and travel. But the same operating system shouldn't be required to run on both tablets and PCs, because it makes the PC version less useful than it should be.
Note: For my full review of the Windows 8 Release Preview, see "Windows 8 Release Preview: Updated but still uneasy."