Despite the superficial similarities of high powered, pass focused Os, with solid Ds that defend the pass better than the run, the Colts and Saints are quite different in style. The Saints offense thrives on the big play. Nearly 1 in 9 of their pass attempts ended in a completion of 20+. They were in the top quarter of the league in runs of 20+ as well. Opposing them is a defense built to take away those long gains. Speed rushers off the edge, coverage LBs, 2 deep safeties and strong tackling DBs combine to force offenses to grind rather than explode. The Colts gave up the leagues' least passing plays over 20 yards. The Colts were in the top half of the league at preventing runs of 20+ despite their total more than doubling (4 to 9) in the final 6 quarters of the season as the backups were gashed for nearly 400 rushing yards in 1 1/2 games. With an effective Freeney the Colts defense can force the Saints away from the big play, to a more difficult pattern of grinding out yardage.
On the other side of the ball the Saints D's mantra is the takeaway. The Saints tied for 6th in the league with 13 fumbles recovered, and ranked 3rd in interceptions with 26. Some interesting research on turnovers would suggest that these rates of "takeaways" aren't sustainable and are unlikely to be the result of a repeatable skill. The Saints may have recovered the 6th most fumbles with 13, but they only forced 15, about the league average. The research on fumbles indicates that holding onto the ball is a skill, ripping the ball loose is a skill, who recovers the ball once it is a fumble? Entirely chance. The Saints have had what the Football Outsiders call "fumble luck". It's one of their quick and easy checks for a team that is outperforming it's actual talent level.
On to the interceptions. The Saints had the 3rd highest rate of interceptions on defense this past season, nearly 1.5% above the league average. Like the rate of fumble recovery, the research says that this is unsustainable. A defenses interception rate doesn't just lack year to year correlation, inside a season there isn't a significant correlation between past int rates and futures ones. This led Brian Burke of AdvancedNFLStats.com to say,
"Interceptions are very random, and they are 'thrown' by an offense much more than they are 'taken' by a defense."
Ignore the talk of the Saints playmaking defense. They don't possess any repeatable skill for generating turnovers at a rate above league average. They've just been on the lucky side of the year to year variations. What the Saints D really is, is a unit that couldn't hold opponents under 21 points a game (20th in the league) despite some of the best turnover luck of the year.
(a less statgeeky version of this post will be available at WaPo's The League, later today).
Update: Shake at The League