The best thing about the original was your Agent's incrementally expanding agility. Other games (inFamous, Prototype, Just Cause 2) may have already explored the boundaries of sandbox agility to the point of diminishing returns.
The other cool thing about Crackdown, as Spike mentioned earlier, was the dead-simple mission structure. No cutscenes, no karma bar, no doomed love interests - Crackdown just wanted to you clean up the boss roster any way you wanted to. I was kind of hoping Ruffian would take the Crackdown formula and just do 'bigger, better, more' without tinkering too much. I do wonder if zombie hordes, beacons, drop points, and orb wrangling will diminish what made Crackdown so appealing for me.