UPDATE 6/22/2010: Re-uploaded all of the concept images. Sorry!
I think the largest issue in terms of usability with the Windows operating system to date is the lack of a central unified control panel. While the OS does have a fairly large control panel, there are still masses and masses of properties, options, settings, etc. that should have been merged into the control panel as well.
Users will navigate to the control panel, only to be lead through a maze of links and additional windows. One might open the Control Panel, open a section, click on a sub-section, click on a link within that sub-section, click on a properties link only to have a new window opened, only to click on another link to open another dialogue with multiple tabs, which might lead to another window, and so on so forth... You can open the control panel and endup in a completely different end of the OS within two or three clicks.
It makes Windows ridiculously difficult to use. Even tasks such as Disk Defragmenter, Disk Cleanup, things like that... there's no reason they shouldn't be a part of the control panel.
I realize this would clutter the control panel immensely, which is why I believe the entire CP requires next to a complete redesign. It's a huge task, yes, but it's one of the largest shortfalls of the OS. OSX's control panel is praised for it's ease of use because it tackles all of these issues. Whatever you click, you're always in the same window that you started from. And now it's time for the Windows OS to tackle this as well.
If you DEMOTE this suggestion, please post an explanation as to why. I think I've covered my reasons as to why this should be done, and it's only fair you post your reasoning as to why the control panel shouldn't be centralized. I'd be extremely interested in hearing why some might think so...
Merge everything, all properties, options, tasks, EVERYTHING into the control panel.
Please see the mockups I have created here: http://davidswebserver.com/ControlPanelConcepts/