I think the most complaint about Win 8 is the Metro/desktop context switch. In other words, people are more comfortable with having everything jammed under the familiar "Start" button, since that is where everything has been for almost 20 years. After using Win 8 for a few minutes on proper hardware, I became very used to the context switch. In a way, it serves as a way of convenient way categorizing apps. Multi-function, multi-window work = desktop. Singular task, single window touch centric apps = metro.
Which would be all well and good if I could stay on the fucking desktop and pick which program to run next. The start menu was seamless in this respect. Bringing up the metro menu is a non-option (far too jarring). So my options are a). 3rd party software to hack back in the same functionality I used to have for free b). clutter the hell out of my desktop/quick launch/taskbar or c). shut up and eat the shit Microsoft is shoveling at us.
Also, it's a complete non-starter in enterprise environments for the very simple fact that new apps only appear on the Metro screen of the person who installed them. So if a tech installs, say, Microsoft Office, then only the tech gets the Office app tiles in Metro. Everybody else has to know that Office was installed, go to all apps, pin it *one by one* onto their Metro screen and fix up their UI before they can use the app. That's not as big a deal for dedicated computers, I guess (though training 1000+ users on how to do that would be a clusterfuck of epic proportions), but on shared computers especially it would be hell. Unless a service pack or update fixes this colossal screwup by Microsoft, I won't be able to recommend Win8 to our company...ever.