Everyone complains about the missing start menu, but that's not what bothers me about 8. I've never cared for the start menu, and I rarely use it in my day to day tasks. I just put icons for frequently used programs on the desktop, so in a way it's a lot like the start screen already.
One of the things that bothers me about 8 are the way they hide frequently used features (like shutdown requires dealing with 3 distinctly different user interface paradigms).
The other thing that bothers me is the complete lack of integration between the two different user interfaces. There's a separate windows update for each interface, two control panels, and so on. Windows update for desktop/OS starts in the new interface, but then takes you to the desktop if you want to view optional updates. Same thing for the control panel, the Metro version only has a few options, so you have to go to the desktop version to do the rest. It's annoying to me, but most people won't even know there's something missing, or what to look for.
Metro feels very unfinished because you always have to go back to the desktop for things. Not even the built in things like the control panel, or Windows update are full featured in Metro. Hopefully they will finish it, so you only need one of everything.