If there is a way to look at this roster and say, it's not bad on paper, then the coach deserves some blame for "underachieving". The issue is complicated but I believe the main problem is that we are too myopic and naive in our approach towards the game.
As the saying goes, you are only as strong or good as your weakest link, and your weakness is bound to be exploited by good teams. Yet, we don't really work on our weakness. Like under DA, we just try to use our strength as early and as long as possible and hope to get by the rest of the way when our aged legs get tired and when second units have to come in without yet a coherent approach for them to succeed in any way.
More precisely, we start with feeding Shaq and keep playing Shaq and Nash and Amare together way too long. With the reserves, we just let Barnes make the erratic decisions. And when Nash puts all his energy "creating" with the ball, he is not as good shooting the ball once he could anytime in the game, nor could he have enough energy to defend better. Without rhythms, besides Shaq's sure points, often nobody gets hot hands at all, and Barnes and LB will keep shooting nevertheless.
Porter's initial idea was sound, in theory, that Nash should play more off ball. This would give the team a chance to work on their weaknesses: scoring without Nash dribbling; getting Nash into good shooting rhythm to be a constant threat this way; getting Nash more energy for defense; slowly giving Amare more chance to direct the offense; work on other set plays that will be needed in playoffs when Nash almost surely gets shut down to create offense. Yet, he is neither confident nor authoritative enough to force his way through. Now, he is just copying DA's coaching: give the ball to Nash and hope for the best by exhausting him.
Guess what I really wanted to say is: Reserve your best for the times when you need them most and work on your weak link first! So, feeding Shaq has now become a sure thing, we should start with something different first, like screening for Nash to catch-n-shoot to get his shooting touch on first, or for Amare to get his. And in time of scoring draught, we could go to this safest set with rested Shaq to gain momentum. Right now, Shaq and Nash get spent way too early without pressing need.