There are many aspects to playing defense. Quick feet, quick hands, anticipation, physicality, timing, etc., are all parts of playing defense. Some players have more of these defensive tools than others, and obviously the more you have, the better of a defender you are. Rondo is known for his very quick hands and excellent anticipations, and thus has been called a good defensive PG, but so far, he is almost ineffective against Derrick Rose.
On the other hand, Marcus Banks is also known (or has been known) for his defense--but not in the same vein as Rajon. Banks doesn't have the quick hands or the anticipation skills that Rondo possesses, but his defensive prowess comes in the form of keeping his man in front of him (for the most part, that is). Banks was good at staying low and making his body wide, and sliding his feet to shield his man from getting his shoulder past him.