If you liked the first one, you'll probably enjoy the second.
Some new things about the second that I really enjoyed:
4 player co-op - I really wouldn't recommend this game as a single player activity. Yes, it is one of those games. But with at least one friend to play with, it's a blast.
Zombies - at first I was like, "Zombies? Really?" But then, when I got in my car and started running over zombies by the hundreds, I understood and said, "Yeah, zombies!" Crackdown was obviously missing its zombie filler.
New abilities - I love the gliding thing. It's kind of awesome.
New Cars - I love the SUV. It's kind of awesome.
Some things people might not like:
Repetition - there are like 3 kinds of missions. And you do them many, many times. But, this makes it much easier to jump into a friend's game without missing a beat.
Single player is not so fun - I mentioned this above, but it's important enough to mention twice.
Graphics - they are not really better. Considering the length of time between these two titles, they should be.
The missed opportunity - The Quacker grenades don't explode with feathers.