Yes typically when I buy a sequel, it's because I want more of what was in the original. It's why I bought resident evil 5, devil may cry 4, halo 3, soon halo reach, mario galaxy 2, smash brothers, and a shit load of other sequels.
Crackdown is a unique game against all others, flawed as it was, the sequel fixes MANY of the problems I had with the original, perhaps I notice because I went for ALL of what crackdown 1 offered.
Stunt rings, orb hunting, orb variety, objectives, weapons, all of it is vastly improved over the original. Want all 500 orbs in the original? Good luck, it's a BITCH and a pain to do, in the sequel? challenging, but they don't leave you in the dark. Get annoyed finding those invisible trigger points for stunt rings? No problem here now. Almost every aspect for the individual facets of the original have been improved, and plenty of stuff has been added.
Now, if you don't want more crackdown, do not buy crackdown 2. I kinda figured this was a given for any sequel, I didn't like littlebigplanet much, and i'm not dying for the sequel.