Lack of mental toughness, which leads them to doing things like folding against the Heat when they should have won (Spurs would have beat the Heat, and were 1 possession away from being there, and have the mental toughness needed to win a championship)
I think Josh Howard may have that toughness, and that may be enough to get them a championship, but I KNOW FOR SURE Dirk doesn't have it. If he did, it would have showed already. Maybe Devin Harris has it too, but that's about it for who has mental toughness on the Mavs. The rest (Dirk, Stack, Dampier, etc) all just either fade or settle for bad jump shots when the going gets tough.
It may or may not be enough for them to win. But also, the Mavs have been incredibly lucky and not survived many injuries so far this season, unlike almost all the other teams. We'll see if they can keep that up.
It's not even All Star break. I'm not ready to crown the Mavs yet. My money's still on the Spurs in another 7 game series, and frankly, the Lakers scare me more, because I think Kobe playing well and Jackson coaching is a lot scarier than anything the Mavs have to offer.