Mike wondered why the system notification icons went colorless in Windows 7 and why they went back to regular tooltips instead of the custom tooltips.
I don't know either, so I asked Larry Osterman, who was in charge of the Volume icon.
And he didn't know either. He was merely given new icons by the design team.
But that doesn't stop me from guessing. (Which is what I do most of the time here, I just don't explicitly say that I'm guessing.)
My guess is that the design team looked at the new Windows 7 taskbar and noticed that all the system-provided pieces were subdued and unobtrusive, with two exceptions: The Start button itself and the notification icons. The Start button kept its bright colors because, well, it's the Start button. But the notification icons? They are peripheral elements; why do they stand out on an otherwise neutral-looking taskbar? Isn't that just drawing the user's attention to something that doesn't deserve attention?
So boom, make them monochromatic to fit with the other taskbar elements. The clock is monochromatic. The Show Desktop button is monochromatic. The taskbar itself is monochromatic. Hooray, aesthetic unity is restored.
As for the return to standard tooltips, that's easy: The custom tooltip was a violation of the user interface guidelines.
The old Windows Vista custom tooltip did not provide any useful information beyond the standard tooltip, so you paid the cost of developing and maintaining a custom tooltip for very little benefit. In the volume tooltip's case, the developers were spending effort fixing little bugs here and there (for example, there were painting glitches under certain accessibility conditions), effort that was detracting from other work that could be done, and switching to the standard tooltip made all the problems go away.