Experience...also the 3 year warranty. Seagate would be my second choice (little experience with their drives), but from my experience they are very quiet and the 7200.7s look fast. I've had bad experiences with Maxtor/Quantum, I won't buy anything from them unless it's revolutionary (á la Raptor).
So to answer your question more directly: the fact that I've never had a WD die on me (I've owned quite a few) says a lot. Their performance is also stellar, and when combined with a 3 year warranty, they are unbeatable, IMO.
Maxtor disgraces the six letters that make Matrox.