After watching a good majority of Suns games this year, I've concluded this:
The Suns are not a bad defensive team. In the last 10 games, in fact, the Suns are holding opponents to the lowest percentage in the league: .425, narrowly beating out Detroit, Utah, and Houston. If you count tonight's game against the Knicks, who the Suns held to under 38% shooting, that percentage is even lower.
On the season, the Suns are holding opponents to an average of about .450, which is good enough for 9th or 10th in the NBA. That's better than Dallas and San Antonio.
The Suns' biggest weakness is defensive rebounding.
They desperately need another rebounder on the roster. Unfortunately, Shawn Marion is being forced to guard smaller opponents, which means he's not in a great position to rebound (since he's guarding on the perimeter). Kurt Thomas isn't getting more than 20 minutes a game, so there's not much he can do to affect this problem. Boris Diaw looks to be more of a playmaker than a force inside, so he's not helping much.
Amare Stoudemire is doing a decent job of attacking the boards, but he doesn't have a nose for the ball like Marion does.
The Suns need to address this problem before the playoffs, or it will be their undoing. If they can remedy this problem and become a better rebounding team, then they'll be one tiny step ahead of both San Antonio and Dallas, theoretically.