When it comes to Google Reader I use "share" for things I like, would like to share with others and find again. I use "star" only for items that are not processed yet (eg. unread essays, unwatched videos) and I remove the star after processing.
This is exactly how I use it as well. I share items I like and want others to see and star items for future action (Youtube videos I can't view at work, article I want to read, SmashingMagazine's excellent graphics/icons that I want to refer to later).
