I think a favorite word saver or storer is in order here. A program that you can enter in your favorite words when you find them as time goes by, and then come back to the program and see the list that contains your favorite words! I think its a good idea. --- The Favorite Word Saver --- needs a better name but other than that, its a good idea.
Nice idea, maybe apple can just enable a 'flag' in the dictionary. That way you don't even have to type them .
Not trying to be insolent but what's wrong with copying and pasting them into a Textedit file? Or do you want to do crazy stuff with them like rate them, sort them and put them in groups?
An idea such as Redeye's would be much more convenient and prove to better use. I think Apple should do something like this. Like the widget manager, but word manager.