There are no dominant defenses this year. You could make an argument that all of the top 10 defenses are just above-average, not exceptional. Washington has been playing solid defense, for example (7th in yardage, 10th in points) and has given up 45 points tonight to the Giants in just over 3 quarters of play. Green Bay had been playing as well as anyone, and got shredded by the Steelers for 37 points and over 500 total yards (over 500 passing yards alone). New Orleans' big play defense gave up over 430 yards to the Cowboys and produced 0 turnovers.
The 2008 Steelers defense averaged 13.9 PPG and 237 YPG, had 51 sacks . The 2008 Ravens defense averaged 15.2 PPG and 261 YPG. Both gave up about 80 YPG rushing on average (80.2 for Pittsburgh, 81.4 for Baltimore), ranking #2 and #3. They ranked #1 and #2 in pass defense, and allowed the lowest passer ratings (60.6 for the Ravens, 63.4 for Baltimore). Those were dominant defenses. No defense this year comes remotely close.
There isn't always a dominant defense. It's rare that there are 2 in one year. There was no dominant defense like those Steelers and Ravens defenses in 2007, or in 2004. Besides the Steelers (2001 and 2008) and Ravens (2000, 2006 and 2008), the only other teams that could come close to claiming to have dominant defenses this decade are the 2002 Tampa Bay team (12.2 PPG and 252.8 YPG), the 2003 Patriots (14.9 PPG and 291 YPG, with 29 interceptions and 29 forced fumbles) and the 2005 Bears (12.6 PPG and 281 YPG).
We're a top 10 defense in a sub-par defensive year. There just aren't any particularly good defenses this year. We can hold our own with Baltimore (lousy secondary), the Giants (lousy secondary, aging LBs, and problems with their pass rush), Pittsburgh (injuries), Indy, New Orleans, Philadelphia, Green Bay, San Diego, San Francisco and Minnesota. Cincinnati and the Jets seem to be better than we are, and perhaps Dallas right now. But no one is dominant.
We're a top 10 defense, and we're pretty average. This year, those two things are pretty much equivalent.