Co-op, new items, and multiplayer should have been there, this pack adds just that...but can be deceiving...
The addition of new items helps by which you no longer have to resort solely to military strength, but can buy seats in a council to gain influence. You must have 3/5 seats to take over the island.
Graphics:
Graphics are very well suited for most computers. Ran it at medium-high settings on 1024x768 on a 9500GT with FPS upwards 35+
8800GTX SC handled it like a champ, no lag whatsoever.
Sound: The music keeps you entertained, however they should have added more variety to the sound track
Gameplay: The learning curve is a bit steep, but definitely rewarding to say the least. After an hour or two, you'll be so engrossed in building your city and keeping up with the needs of your people, time will fly!
There are no major gameplay changes besides those above mentioned. For $20, I would say definitely worth it for the multiplayer!