The new Aperture version handles previews differently. You do not actually need to create previews to share your images to native Apple applications, like iLife and iWork applications.
See this discussion Major change to previews in 3.5?
and this support article: http://support.apple.com/kb/HT5932
But still, "Quick Preview" does disable the adjustment pane, at least for me- like firstlaunch said.
Are you sure, that you are in "Quick Preview" mode? Is the "hurried square" icon highlighted in yellow?
Maybe simply your "P" shortcut has become disabled. Does the "Quick preview" menu entry in the "View" menu still show the shortcut "P" or is it gone?
If not, fix the shortcut from the Aperture > Commands > Shortcuts menu.
If the shortcut is still o.k., and you are in Quick Preview mode, test this issue in a different user account, before trashing your preferences, like first launch suggests.