This game was oddly rushed based on interviews like Tom Chick's and building the new engine didn't allow them to go all out. Plus they were working on the Red Faction games at the same time, now they can just focus in on adding all sorts of awesome. They have the engine in place, the city is generally built already and they can add onto/rework it like Stilwater was for 2, all the other improvements to the gameplay are in place.
I mean there are all sorts of things like hospitals, arenas, casinos, warehouses, etc. already around the map that aren't used. Everything downtown. SR2 took you into non-cribs like the museum, hospital, the rec center, etc. for missions. In SR3 you basically only drive by any of this, maybe start/end a mission outside one.
I assume most of the DLC is actually cut content. (Intentionally so or not.)