Dawn orbits Ceres at thousands of kilometers periapsis altitude, and doesn't seem to plan to go lower than 375 km. In contrast, Rosetta has been down to about 30 km altitude, and will finally even try to soft-land. And that's at a comet with a coma that might already have caused Rosetta to enter safe mode as its star tracker confused debris around it for stars.

Is it maybe because of gravity, propulsion, instrumentation or spacecraft design? Isn't closer better?