Building on top of an already strong foundation.
The very few things that could use some improvement include the resetting in difficulty in the horde game where the enemies start off rather simple and get significantly harder as the level number progresses to and of the 8,9 or 0 of fthe next set. The only other issue is the lack of answers about who, what, where and when in terms of the story. The questions from the first game are not even answered.
Overall this is a must play for anyone who likes shooters. It is great to play with your friends and it supports up to 5 player co-op!