Intended features reasons to be included are not easter eggs. From Wikipedia:
A virtual Easter egg is an intentional hidden message, inside joke, or feature in a work...
Therefore Aero Shake is not an easter egg.
There are no longer any officially sanctioned easter eggs.
Microsoft formally stopped including Easter eggs in its programs as part of its Trustworthy Computing Initiative in 2002. - Wikipedia
While this policy is in effect, any easter eggs that make their way into the product would be the result of a rogue developer.
One of the aspects of Trustworthy Computing is that you can trust what's on your computer. Part of that means that there's absolutely NOTHING on your computer that isn't planned. - Larry Osterman