The New Orleans Saints, in a very unexpected move, fired defensive coordinator Steve Spagnuolo after only one (turmoil filled) season.  I think it  may be a bit premature, as he's a highly touted coach.  This was obviously a season with extremely extenuating circumstances, with a lot of things that were very out of his control, and Spags may have taken the fall unfairly. I am of the opinion that Steve Spagnuolo may be the best 4-3 defensive mind in the league since the passing of his mentor, Jim Johnson.

With that out of the way,  I want to discuss and break down what the move to a 3-4 defense means for the Saints moving forward, and the players currently on the roster.

Most people know the most basic difference between a 3-4 and 4-3 defense.  A base 4-3 has 4 down linemen(ends and two tackles between them), and 3 linebackers.  The 3-4 is the reverse, 3 down linemen (2 ends and one nose tackle)and 4 linebackers.

On the surface, it doesn't appear much different, but the biggest difference that most won't recognize is the basic function in  the front 7.  In most cases, in a 4-3, the down linemen, particularly are meant to function as pass rushers and penetrate to disrupt plays in the back field.  The linebackers are usually guys who can move well sideline to sideline, drop into coverage, or blitz open gaps.  The 4-3 usually uses a  1 gap system where the linemen attack a single gap(a gap being a space between two linemen.)

In most 3-4 defense's, instead of the linemen having a specific gap they attack, each of the 3 linemen engage 1 to 2 blockers, and have 2 gap assignments.  The main priority of the linemen isn't to shoot through gaps and attack in the backfield, it's to occupy the blockers and free up the linebackers to rush or clean up tackles inside.  This chart shows the basic gap assignments in a base 3-4 (though this obviously changes drastically when blitzes, stunts and coverages change.)   The defensive back assignments are basically the same in a 4-3, they won't be shown:

As you can see, each gap has two defenders assigned to it, so if one loses the battle with the blocker, there is another guy there to make a play.  In this scheme, the outside linebackers(OLB) will be much like the defensive ends in a 4-3.  They will be looked at primarily as edge rushers who look to get after the QB,and occasionally drop into coverage.  The two inside linebackers(ILB).  As mentioned earlier, the defensive ends(DE) and nose tackle(NT) will be primarily space eaters, working to take on the initial blocks and allow the linebackers to make the plays.

Neither defense is inherently better, it all  depends on coaching and execution from the talent, but generally the 3-4 makes it easier to disguise where your pass rushers are coming from, and allows you to move linebackers to different spots, giving more chances to overload certain sides, where the offense has less blockers than the defense has rushers.

Even as a fan of Spagnuolo, changes clearly had to be made. I think it's a good idea to move to  the 3-4 this season, if we were going to.   The Saints gave up the second most points in the league (31st out of 32 teams) and the 7000+ yards they gave up on defense was the worst in NFL history.

The good news is, some of the younger and more talented players on the roster are guys who could potentially be excellent fits, and building blocks going forward.  What does it mean for our defensive personnel on a whole though?

Nose Tackle:  Sedrick Ellis and Broderick Bunkley were the starters at defensive tackle this season with Akiem Hicks rotating in.  Ellis is a classic 4-3 under tackle, a solid player, but he's a free agent, and not a 3-4 fit, and I expect him to be let go.  Bunkley has functioned as a 3-4 NT in Denver and can be solid up front.  This is arguably the most important position in a 3-4 defense, with teams looking for a dominant space eater that demands a double team and threatens to wreck any quarterback or running back he can.  These are your Vince Wilfork and Haloti Ngata's(Though Ngata has  played a lot more end recently) of the league.

Defensive End:  Will Smith is another classic 4-3 player, also with a huge contract that I expect the Saints to release, as he would be a poor fit going forward, and his productivity and health continues to diminish.  Cameron Jordan is terrific 3-4 defensive end prospect.  He has the size at 6'4, 295 lbs to take on blocks, and the athleticism to generate pressure(even getting 8 sacks in the 4-3) or occasionally drop back into coverage.  I expect him to thrive with the scheme change, as he also played it in college, and he could potentially be a pro bowler.  You want guys who can apply some sort of rush  while also taking and shedding blocks to make  plays in the backfield like JJ Watt or Justin Smith.  Controlling the gap is the main responsibility, anything else is just gravy.

Wild Card:  Akiem Hicks is a huge(6'4, 320lbs) and raw athlete with an immense amount of potential.  He still plays too high to anchor as a nose tackle in the middle, and he has some  technique issues, but he could be the type of versatile linemen that teams like New England covet, potentially playing at defensive end and tackle in the scheme, causing  havoc and confusion all over the line.  I expect him to stat as an end, but a few years and a few pounds down the road, he could be a terrific nose tackle or end.

Outside linebackers: Martez Wilson is another guy that seems to potentially be an excellent fit in the 3-4.  He was already getting reps at both defensive end and linebacker, so he seems tailor made as a hybrid pass rusher, and he can drop off into coverage, or chase down and blow up screens before they start.  He has amazing athleticism in a 6'4, 250+ pound frame.  Junior Gallette seems like a better fit  as a 4-3 end than in the 3-4 scheme, but he could function as a pass rushing OLB, but I'd be concerned on whether or not he has the coverage skills to thrive. These are the primary pass rushers in the defense.  These guys attack on the edges, on the outside of the tackles, much like ends in a 4-3, looking to force QBs to get rid of the ball, or get squashed.  The ability to drop into coverage adds a lot of flexibility for a coordinator, and confusion for an offense.  The type of player you're looking for is usually in the mold of Aldon Smith, Demarcus Ware, or Clay Matthews.  Guys like Chris Chamberlain, Jonathan Casillas, and Will Herring are poor fits in the scheme and should be expected to be released before long.

Inside Linebackers:  Curtis Lofton and David Hawthorne are  excellent and taking on blocks and still making tackles in the box, and will be solid fits inside.  Lofton in particular should thrive in the "thumper" role, aggressively attacking against the run.  Neither has the athleticism to really excel when rushing the passer, and could be subbed out on obvious passing downs (particularly Hawthorne).  Martez Wilson is versatile and strong enough to slide to these positions and generate a rush, and I expect the Saints to go for another guy that can shed blocks and drop into coverage.   Jonathan Vilma was traded to us because he was a poor fit  for the Jets new 3-4 scheme, and considering he's lowing down and has a big cap hit, I don't expect him back.  Patrick Willis and Navorro Bowman is an excellent example of great synergy and skill set combination for your inside 'backers.

As mentioned earlier, I don't expect Will Smith, Sedrick Ellis, or Jonathan Vilma back, and if they are, they probably won't be huge contributors on field.  I expect the Saints to draft heavily in the new scheme, and take a pass rushing OLB with our first pick.  My projected opening day starters,  very early in the process, and subject to much change obviously:

DE: Cameron Jordan
NT: Broderick Bunkley
DE: Akiem Hicks

OLB: Martez Wilson/Junior Gallette
ILB: Curtis Lofton
ILB: David Hawthorne
OLB: 1st round pick

Small nuances could potentially very different depending on who the Saints hire as the coordinator.  There hasn't been much talk yet, but the top candidate I would look to would be Romeo Crennel, who coached championship defenses in New England, and made very quick defensive turnarounds on teams like Kansas City.  A dark horse can be someone like Kirby Smart, who some will recognize from his tenure at LSU, and helping orchestrate the dominant defense in Alabama since 2007.

It will be odd seeing a 3-4 defense, as we haven't run the scheme in well over a decade now, also seeing such big names potentially walk will sting a little, but now is the time for change, after an unprecedented and very disappointing season, moving to a new scheme is a welcome change and should bring youth and optimism to a very disappointing defense.