Setting aside talent and record, my guess is that the worst NBA squad ever, whichever team you'd like to peg for that unpleasant distinction, was probably characterized, among other things, by a lack of effort or cohesion on both ends of the floor.
The Sixers go into every game at a talent disadvantage, and clearly working in Okafor at center has torpedoed their previously solid defense, but whenever I've watched them it looks like they're at least trying. They play hard and fast, attacking the rim and trying to force turnovers. They just don't have enough real NBA players to win that way.