If you haven't already seen it, make sure you check out Cold Hard Football Facts, a great stat/info site on the NFL. Probably the most interesting part of the site are the Quality Standings. Take the Bendability Index for example:
This is the first stat that chronicles the phenomenon of the "bend-but-don't-break" defense and provides a measure of defensive efficiency. The Bendability Index is obtained by dividing a team's total yards allowed by total points allowed, yielding Yards Per Point Allowed. A team that ranks high on the Bendability Index has the defense that opponents must work hardest to score upon. This effort is more important than total defense and, in many cases, more important than scoring defense. The Bendability Index is not purely a defensive yardstick. It is, instead, a great barometer of team success. It is a function of many team-wide factors, including general defensive strength, offense and special teams proficiency, turnover differential and Red Zone defense.
Don't get in too deep. You'll end up spending all day pouring over those stats.