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The Giants suffered a historic defensive collapses in the second half of the 2009 season.  Only the Rams and the Lions allowed more points.  The highly touted defensive line failed to get to the QB with any regularity and was soft in their run defense.   The Giants and fans had high expectations for the D-line.  Packed with talent it was expected to rough up QBs and make it tough to consistently run the ball.  Between injuries (both past and present), age the underachieving unit failed to perform anywhere near expectations.   The failure to provide a good pass rush exposed the secondary, a secondary that was handicapped with a horrible safety unit, as a result of Kenny Phillips being lost for the season.   Add a linebacker corps that was too slow to stop the run or the pass and you had the makings for a defense that was painful to watch.
There are two primary areas of blame.   Injuries were undeniably one of the main culprits.   The injury bug bit Canty, Ross, Phillips, Pierce, Tuck, Boley, Alford, Bernard, Sintim, Wilkinson, and slowed down Cofield, Robbins and possibly Osi.  Few NFL defenses would be able to weather that many key players injured.  To make matter worse the team's roster lacked any depth at Safety which multiplied the effect the loss of S Kenny Phillips had on that unit.  
The second trouble area was with the new DC Sheridan.  While Sheridan proved an invaluable asset to Spags (playing the role of the Xs and Os wizard up in the booth, on the other end of the headphones) he proved to be less than ideal as the coach in charge of the defense.   He lacked the ability to motivate or even lead the defensive unit.   His dispassionate intellectual approach to the game did not play well with the players.   Sheridan also seemed to be poorly prepared in some games and was clearly out coached in some games(the most glaring example would be the New Orleans game).  The lack of leadership seems to have allowed infighting to flourish.  In the end Sheridan was fired and Wauffle, long time Giants D-line coach was also released.    
The team replaced the Departed Bill Sheridan with former Bills DC and interim head coach Perry Fewell.  While not known for innovative defensive strategy,  his active and vocal style provides good motivation and accountability by the players.  He is well respected by the players that have played for him.   He could best be compared to former Giants DC John Fox.  

Unit by Unit Breakdown: 

DEFENSIVE LINE

Defensive End- The left defensive end position has the Justin Tuck as the projected starter.  When healthy he is Pro Bowl caliber starter who is stout against the run and adapt at getting to the QB.  The problem is his injury history.   Injuries have had a significant impact on 3 out of his 4 seasons in the NFL  When healthy he is one of the best, but that is far from a sure thing.  Beyond the injury issue, the team also likes to move Tuck inside to a tackle position on passing downs.  
More than likely the team hopes that rookie first round pick,  Jason  Pierre-Paul will prove to be the first backup to Tuck.  This is a bit of speculation on my part, but I think his size (270 pounds with room for more) makes him a logical choice.  If JPP proves to be too raw for anything but part time pass rush specialist the backup role will most likely be filled by Kiwi. 
The right defensive end position is a more interesting situation.  Last season Osi Umenyiora lost his starter's role due to his poor play against the run.  While never the strongest run defender Osi seemed to take a step backward last season.  Some writers and bloggers have speculated that his failings were the result of his prior season's knee injury, I never heard it mentioned by the coaching staff or by Osi.  I suspect is was more an issue with the coaching staff that is no longer with the team.  Replacing Osi in the starters roll was Mathias Kiwanuka.   Kiwi did an adequate but unspectacular job as Osi's replacement.   Both men have stated that they feel they deserve to be the starter this season, so this should be an interesting and hopefully non-disruptive competition.  Kiwi is also in the last year of his contract and Osi is hoping for a better contract than the one he was unhappy with the prior 2 seasons.  So there is a strong financial incentive for both players.
Rounding out this group is Dave Tollefson , a journeyman caliber DE.