I think the operative concept is not toughness or softness, it's agression. I for one remember a few years ago when this defense was flying around smacking players for 60 minutes. Wilson was a concussive hitter, Harrison was Harrison, Bruschi was smacking people in the mouth and taking the football from them, Ted Johnson was breaking guards helmets, hell, even Samuel was laying lumber in playoff games. There was an air of intimidation and intensity around this defense in big games. Remember when this defense would swarm to the ball? Remember the emotion of the 03 game against Indy? Remember how impressive that level of physicality was? It seems that is lacking lately.
Harrison still tries to lay wood, but he's arriving to the football a step or two late. Bruschi just isn't anywhere near as explosive a player as he once was. I think James Sanders is the poorest hitting safety I've seen in a while. Meriweather tries to knock helmets off, but somehow seems to lack the momentum behind his pops. It seems that this team lacks the overwhelming intensity it once had.
I understand that defense isn't all about intensity or ferocity, but it is the trademark of a top-flight defense. Anyone who has played on the defensive side of the ball and really loved hitting has had a contempt for offensive players. A defensive player needs to have an air of agression about him. He needs to play in control, but remain nasty when playing. Most offensive players don't have the heart for a 60 minute street fight. It's critical to set the tone early with the specialists. Specialists as a generality are soft by nature, they shy from contact. If they feel that they are in for a war, they will often loose their desire to compete.
Scheme is great, in fact I think that is what defines a great defense. That said, a defense needs the players capable of playing with a screw loose. A good defensive player takes pride in making a tremendous hit. It's like hitting a homerun; you line up the opponent, drop your hips, explode from the balls of your feet though your hips, see a blinding white flash and feel no impact. You look around and your opponent is sprawled on the turf, momentarily motionless. That is the greatest feeling in football, and this team needs more of it.