I would pay about 500 USD for having the XP interface on Windows 7.
And for what it's worth, XP has configurable systray settings. They are "Always show", "Always hide" and "Hide when inactive".
The biggest features I miss, in order, are:
(1) The XP start menu - I had it organized just perfectly for muscle memory. If I want to start apps by typing I can use the console, thanks. Start Menu Search is just fine, but I don't use it.
(2) Windows Explorer. I have my toolbar / menu / location bar in Explorer in XP tuned for maximum vertical space. The folder treeview in XP works correctly, unlike the buggy one in Win7. Here's a demo for you: open up Explorer with the treeview visible. Select a folder that has subfolders. Then expand the subfolders by clicking on the little nub next to it. Observe that the treeview scroll position jumps so that the expanded node's children are aligned to the bottom of the visible area. This drives me up the walls. Other things include the giant file drag icons, the all or nothing approach to thumbnails, the file locks held on Thumbs.db on folders across the network (prevents deletion from Explorer itself!), the weird listview selection semantics, the lack of a Favorites menu, the way the treeview disappears when the Control Panel is selected, the lack of a selectable Network Connections submenu / subfolder in the Control Panel shell namespace, etc. I could go on.
(3) User colour preferences are not respected. I like my sky blue window background, and in XP it is correctly used for the treeview and listview in Windows Explorer. Not in Win7 - no, it's glaring white everywhere. It's interesting to see how inconsistently the user preference for window background colour is applied. Lots of text areas sometimes show white, sometimes the user colour, depending on their focus state, the application, etc. There's no rhyme or reason to it. It scans like a shoddy joke.
I guess I should clarify one thing: the XP interface I use is not the Fisher Price one that it comes with out of the box, but rather the Win2000-alike one but that still has the XP start menu and other XP-level accoutrements .