The paragraph above sums it up nicely.
I think the wild card that some may be overlooking is turnovers.
Lets look at the two extremes:
Fantastic Offense w/ mediocre defense - The offense is prolific and racks up yards and points, but has a greater potential for giving up a turnover than a catious, ball-contol style offense. Meanwhile the defense is typically on the field longer because the offense scores so quickly. This is especially true if the offense is geared more for the pass than the run. Tired defenses are less likely get turnovers and more likely to give up a lot of points... something the Colts struggled with prior to last year and the Rams struggled with from 99-01.
Fantastic Defense / mediocre offense - A top notch defense not only does well preventing or limiting points, but often forces turnovers leading to defensive scores or great field position for the offense. In this scenario, the offense must have a half way decent running game (not great - see A.Smith) rather than emphasizing the pass (although a conservative short pass attack like we had in 01 can sometimes work similar to a running game). In this scenario, the offense still has to be good enough to methodically move the chains and not just go 3 and out all the time. However, geting TD after TD is not necessary. Chewing up time, gaining field position, and scoring an occassional FG is usually all that is needed. In this scenario, the offense is less likely to turn the ball over.
Of those two scenarios, I see #2 as being the one that is usually more effective at winning in the playoffs because turnovers are momentum killers, give the opposing offense a short field, or worse - lead to defensive scores.