A lot of times a good NFL defense doesn't translate into a good fantasy defense. A good example of this was Jacksonville. In the past there have been very good NFL defenses that sucked for FF owners, such as Dallas and Carolina. Hitting a a great FF defense is about a 90% crapshoot.
Team defense is about big plays unless your league scoring is heavily weighted towards points given up. To me it boils down to the following things:1) Can the defensive line generate pass rush by itself, without blitzing? If so, there are more hits on the QB generating fumbles, and more hurried passes generating INT's, not to mention sacks of course.2) Does the defense tend to play with a lead or play from behind? If the defense plays with the lead a lot, it will tend to force offenses to take risks they wouldn't otherwise take and the defense essentially gets "bonus" points that don't necessarily directly reflect the overall quality of the defense. 3) What scheme does the defense play? Scheme dictates a lot for a defense. Cover-2 defenses don't tend to be great fantasy defenses because they don't tend to blitz much (the exceptions being those cover-2 defenses that can generate pressure without blitzing). The 3-4 defenses favored by Belichick and the Steelers OTOH, with their confusing looks and ability to bring rushers from virtually any angle, are better because they generate more turnovers. 4) What is the quality of the offenses and opposing QB's within the team's division? This accounts for six of 16 games on the schedule. The Rams in years past benefitted from being in the weakest division in football and got six games against miserable 49'ers and Cardinals teams, and merely fair Seahawks teams. The Bears over the last couple of years have feasted on a very weak NFC North. If you've done your homework enough to answer these questions, you'll generally only be surprised in the event that injuries (either to your team's defense or its offense) greatly affect personnel, but then that's always the case.