Okay, so I figure I’ll get things started here with a little IDP intro, since a ton of people haven’t gotten into IDP.
I actually only started IDP last season, but I’ve ALWAYS been a defensive fanatic (I was one of the few people who actually could name the 89 Vikings sack beasts off the top of my head) and of course, a STAT fanatic, so this was pretty easy for me to pick up.
Anyway, there are 3 “basic” types of scoring, although there are extreme variations even within the basic types. The style of scoring is most often and most easily identified by simply looking at the points for sacks vs. the points for solo tackles.
1) Tackle-heavy – typically, solo tackles will be worth about half as much as a sack (ratio is about 2:1). Look for strong safeties, look for Cover-2 cornerbacks, and, as always, look for MLBs, ILB and WLB. DL are mostly interchangeable
2) Big-Play – typically, solo tackles are worth at most 1/5 as much as a sack. This makes guys like Merriman far more worthwhile, and makes guys like Ware and James Harrison into complete studs. OLBs like Suggs and Woodley are big in leagues like this, as are interception/pass defensed specialists. Run-support safeties like Gibril Wilson and Eric Weddle lose their value immensely, while all-around studs like Atogwe are huge. T2 MLBs like Urlacher who dropped into coverage frequently while still maintaining solid tackle numbers are also gold
3) Balanced – typically, solo’s are worth about 1/3 as much as a sack. There is more value to be had, but the safest bet is to draft your team similar to a tackle-heavy scoring league, while adding a “specialist” here and there. Guys like Woodley, who probably won’t get 80 tackles during the season have value as situational starters because their ability to put up 2-3 sacks and force a fumble or two can help win games whereas solid run support guys like Kirk Morrison will form a great foundation upon which you can build your IDP.
Variations to look for
1) Look for passes defensed scoring – some leagues leave it out, some give up to 2 points / PD. That can make an ENORMOUS difference, especially for MLBs who drop into coverage frequently in a Tampa2. This also boosts the value of T2 safeties who tend to provide more value with the additional PD scoring
2) Solo vs. assist – some leagues don’t give points for assists. This can impact the scoring IMMENSELY, and really brings the IDP scoring closer together. It almost makes drafting anyone other than a basic starter, almost silly and valueless.
3) Crazy points for interceptions. Most leagues give +1 or +2 points above sacks for interceptions. Hint – this year’s interception leader will NOT lead the NFL next season. Poor saps who drafted Antonio Cromartie in 2008 learned that lesson the hard way.
Some very VERY general value-based decisions that can help you draft
these are VERY generalized, and a are designed mostly for a balanced league
MLB > 4-3 WLB ~ 3-4 WILB > > > 4-3 SOLB > 3-4 SILB > 3-4 SOLB (exceptions, based upon scheme – see Dick LeBeau and Lamarr Woodley)
the “w” stands for “weakside”, i.e. the side opposite the TE. 4-3 defenses like Dungy’s Tampa-2 are designed so that the WLB is left wide open to pursue the ballcarrier, whereas the Strongside LBs “block” the blockers, opening up the WLB and MLB to do the pursuit and playmaking
3-4’s can produce some solid #s for the weakside as well (think Patrick Willis, James Harrison, James Farrior, and Terrell Suggs). The DL on these teams are space eaters rather than playmakers (a la a 4-3 DE like Jared Allen or Robert Mathis). Their primary job is to gobble offensive linemen, allowing the LBs to make all the plays. Wonder why a team like Pittsburgh always seems to provide an ideal situation for their LBs to excel?
Casey Hampton REQUIRES a double team. Figure a C and an OG. Aaron Smith REQUIRES a double team. Figure the RT and TE. On the other side, Keisel will do his best to gobble up the LOG. That leaves Woodley on the left side dealing with a RB or possibly a pulling guard, and James Harrison on the other side, with some poor slob LT trying desperately to get away with holding and facemasking.
Now, who is left to block James Farrior? Answer – no one, to the tune of over 100 tackles this past season.
Ideally, you look for opportunity along with position. San Francisco has hardly had a ball control offense, and as a result, Patrick willis has been a tackling machine. NORMALLY, Pittsburgh has such a solid ball control offense that their LBs don’t get a ton of tackles. This past season, the Steelers’ run offense stunk, and Farrior visited the Pro Bowl as a direct result. Guys like Jon Beason have proven themselves to be truly elite because they produce despite Carolina having an incredible rushing attack.
For DBs, it is really scoring-dependant. SS and T2/C2 corners are king, although FSs are truly coming up in value because of the switch to pass heavy offenses. Look for the rookie starters, and look for guys opposite stud corners. Nnamdi Asomugha will ALWAYS produce fantastic fantasy stats….for his starting CB teammate! Why? Because who the hell is stupid enough to throw at Asomugha? NOBODY, they throw at the OTHER CB, so he gets a ton of tackle opportunities.
For DL, it’s mostly a crapshoot. Avoid 3-4 DL and especially 3-4 NT because they typically are in charge of “blocking the blockers”. They aren’t the playmakers. 4-3 DT can have tremendous value, depending upon how they are lined up. Some defensive schemes are designed to free up a 4-3 DT against an OG 1v1, and this is a mismatch that the DT should always win (think Albert Haynesworth, Warren Sapp, and the “original King” of this type of scheme, Joe Greene)
Another thing to pay close attention to is who comes out in the nickel defense. MLB and ILB who aren’t great in coverage will have their valu decrease tremendously if they aren’t on the field during passing downs. Again, a guy like Urlacher was gold because he could cover the pass as well as play the run extremely well.
ok, well, it’s extremely late and I’m tired as can be, so I’ll stop rambling for now. 
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Stuff like this is included in rankings but it might change within a season.
