The limitation of pt. differential is that it is against all the teams in the league, not just playoff teams or teams your going to face in the playoffs.
It is a significant factor in the relative strength of teams, but you need to consider things like experience and individual match-ups and coaching to get an idea of who has the edge in a series. Head to head experience is the best, but it too has limits, as you noted with the Spurs and Cavs last year.
But there's something to be said about that too, namely that the Spurs don't play as hard during the regular season- another sign of old age. As long as they make the playoffs, they think they can win it all. It's hard to know just how good some teams are going to be until it comes to a game they really need to win. They weren't worried about LeBron during the regular season, but in the playoffs it was entirely different.
You know that Boston took them seriously when they played them, even without KG they wanted to send a message. The Spurs weren't exactly themselves, but you would have to think that this would bother them a bit.
In comparing NO vs. the Spurs- there's less than a 1 pt. difference between the two. You mix in the Spurs experience and their defensive ability and it's pretty tough to say who has the edge based on stats. You add in the Spurs slacker attitude and you just can't say. But let either of those teams play Detroit, right now I'd say they'd go down.