Michael has post worth reading on competitive differentiation. The other that I would add to the list is Brand/Marketing Leadership. There are plenty of OK technology products that thrive through differentiation via the brand and marketing effort. More than often there is a tight correlation to product as well. As Warren Buffet said:

…the competitive advantage of any given company and, above all, the durability of that advantage. The products or services that have wide, sustainable moats around them are the ones that deliver rewards…

Durability comes not from one lever but from many. Look no further than the Apple iPod which combines multiple levers for competitive differentiation.