One of Bill Walsh's most insightful football maxims is that a team cannot hide defensive weaknesses for very long. He realized through his own experiences that offensive playcallers are experts at isolating and exploiting defensive flaws, and there were a limited amount of adjustments defensive playcallers could make to prevent this from happening.
At first glance this adage might not seem to apply to the Seattle Seahawks. According to ESPN Stats & Information, the Seahawks' defense ranked first in the league in yards allowed per play (4.42) and per game (273.6), and placed first in four important passing defense categories (Total QBR, passing yards per game, net pass yards per attempt and interception percentage).
The issue for Seattle is that as great as their overall defense and passing defense numbers are, those figures in some ways mask a potentially huge Achilles' heel in the area of run defense. This limitation is significant enough that it could cost the Seahawks a chance at making it to or winning Super Bowl XLVIII.