I don't think such definitions work. EDIT: just realized this thread is from 2015! Anyway, I believe here's where GPs and EPs stand atm.
One could say game packs are always focused on just ONE particular idea. For example, Dine Out is only about restaurants and Spa Day about spa related activities.
EPs are much broader. For example, Get Together's main feature is the club system. But then, there are randoms things like walk-in-closets, cafes and pool venues thrown into the mix, which at first glance dont seem to directly relate with the main feature. City Living is another broad EP; it touches upon different aspects of metropolitan life, eg- festivals, and city based careers with other randoms additions like karaoke and bubbleblowing. Same goes for the bakery and photography skills in Get to Work. So far I believe Cats and Dogs has been the most focused EP; every new feature in it is related to pets: Create a Pet, becoming a vet and running a vet clinic, pet training skill, etc.
So as I see it, GPs are always focused whereas EPs can either be focused or they can compromise depth for breadth (City Living....and the business aspect of GTW IMO suffered because of this trade off that the devs had to make because of budget and deadline issues).
