It would be great to copy and paste elments (like a menu bar for example) between pages in a project. It would also be nice to have a user created library of comon elements in a project that you could drag and drop on a page.
Both of these are great ideas, and you're not alone in wanting them - here's an Idea post created a few months ago requesting similar features: http://forums.adobe.com/ideas/1164
Go vote it up so that it catches the eye of an Adobe staff member!
You could always submit your own Idea post as well.
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