The general new Taskbar concept is fine, but I think it feels only half-done
1. Why are the Notification area icons still so small, it is very difficult to tell them apart and see what they are?
2. If you click on an app icon that is not running or has one instance, the windows maximizes, if you click again, it minimizes. That's good. Why is the behaviour radically different when there are 2 or more instances of an app running: you can no longer minimize any window and clicking does nothing else but bring up the quick preview button. Why doesn't click just open for example the most recently used window instance of that program, and clicking again would minimize that or all windows in that program group? To me, that would be a much, much better usability.
3. I find the Start menu has a lot of redundances and unnecessary noise. Why are there so  many, many links in right hand pane that all lead to almost the same Library place in Windows Explorer.  Why not just eliminate the  all and replace them with one large button labled "Libraries". Simple, effective, easy. Why are there 2 places to pin icons, start menu and task bar? I find it confusing and inconsistent, since your most important apps should now be on the Taskbar.  Why not eliminate that stupid "All Programs" link, and instead directly just show all programs? Fewer clicks = happier users.
4. The replacement for the mini taskbar media player has missing functionality, functionality I use a lot:  no skimming, no volume control.
5. Why does the running  Windows Live Messenger have 2 windows instead of one when hovering or clicking,  making it super akward to launch the list of contacts online (as I mentioned in point 2)

So sorry, I am not getting this new Windows 7 Taskbar in its current form. I find it akward and requireing a lot more clicks and moving the mouse around than the current Taskbar.