The three League leaders in Goal Difference this MLS regular season past were San Jose at +29, the Seattle Sounders at +18, and Sporting KC at +15. In San Jose's case, they virtually lapped the field in terms of goal scoring, being the only MLS club to average more than 2 goals per game and enjoyed a huge gap of 13 more goals than the next highest scoring L.A. Galaxy. With a league average of 45 goals scored per team, San Jose's defense was considerable given the 43 goals allowed - demonstrating the team still played decent defense despite their prolific goal-scoring.
Sporting KC allowed the league's fewest goals, with 27, but were also slightly below average on offense, having scored just 42. With all due respect to the quality of players in Kansas City, it could be argued that Sporting's defensive success was largely system-driven.
In Seattle, however, there seems to be a slight disconnect between reality and perspective, as the team allowed 33 goals on the season, good for 2nd-best and just 2 better than Real Salt Lake - who scored a very average 46 goals. Seattle's attack garnered 51 goals, good for 5th-best in the League and solidly above average; this a season after leading the League in goals in 2011 and adding Eddie Johnson to the mix. The Sounders attack, according to the simple, hard numbers, was solidly above average (+6) while the defense was -12 against the average. And yet, the nearly season-long theme for 2012 was about the Sounder's shortcomings on defense, which is doubly odd considering the season long theme for 2011 was the Sounders attack lacking a "target forward" despite having no trouble scoring goals.
The general consensus, at least amongst those who have gotten past the shrill finger-painting and scapegoating of individual players, is that the undoing of the Sounders defense was so called "mental breakdowns", or lapses in concentration which proved to be the team's defensive achilles heel. When you consider that with Michael Gspurning in goal the side never conceded more than 2 goals in any match - except for the disastrous first leg against L.A. in the playoffs - it certainly seems as though the defending this season was at least consistent; which contradicts the dominant narrative regarding the Sounders defensive lapses.
Soccer is a game of bad breaks, lucky bounces, and wondrous moments. That a team would primarily concede goals on wonder-strikes and defensive mistakes might actually speak volumes about the quality of a back 4. Humans, you see, are troublingly prone to error - much to the chagrin of managers and supporters. In a game with so little room for error, the amount of goals scored off of defensive lapses is a significant part of the game. This low margin of error makes defending a task which requires as much mental stamina as physical.
Pundits love to break down how defenders screw up and concede goals, as there is always a convenient culprit to fall back on. The great scourge of pundits everywhere, zonal marking, is a favorite whenever an attacker finds a seam and exploits it. Strict man-on-man marking breaks down when a defender gets beat and gnashing of teeth of rending of garments ensues when replays show the defender "losing his mark". The zonal vs man marking is an axis upon which there will always be a finger to point.
Another way to look at it is reactive vs. proactive defending. When you hear a pundit complain that a team is "sitting back and giving up too much space" then it is reactive defending that is too blame. When you hear complaints about defenders "being out of position" they are often criticizing proactivity. In the end, you're damned if you do and damned and damned if don't.
In reality defending exists along a two-dimensional map: zonal vs.man marking on one axis and reactive vs. proactive on the other. The mental calculus defenders must perform on every read they make is as important to their success as any physical component. Soccer resists micro-management. Tackle-football coaches map every detail of every player on every play; soccer affords no such luxury. The best approach to defending in soccer will always be the most liberal: a strict system may work quite well but can leave a team exposed to matchup problems against certain opponents. It is far better in the long run to field a capable back 4 and let them play.
There is always room for improvement, of course, and the 2012 Sounders defense is no exception. But when 2/3rd's of your second-best GD comes from stopping goals instead of scoring them, it is hard to really be that critical.