I wasn't really up-to-snuff when the talk of the 3-4 came about this year, so I enlisted a few people to "school" me a bit. Here's what I heard:
What we talk about today is mostly multiple defenses in the sense of what type of defense is being used. You will, for example, see four man fronts quite often used by 3-4 guys. What you won't see is three man fronts used by 4-3 guys as often. The reason, I was informed, is because the personal used in the 4-3 doesn't translate very well to the 3-4. Conversely, the personnel used in the 3-4 is more adaptable to 4-3 if it isn't used constantly.
When the 3-4 is used, personnel is much more critical. Also, you need good (at the very least) players everywhere in the 3-4. It is much harder to camouflage inherent weaknesses in this type of defense. You don't t need great players everywhere, BUT you absolutely can't have poor players anywhere.
You also need depth in the 3-4 regardless if you use the "bend-but-don't break" style, or whether you use a more aggressive style. Players get a workout in this style, and open field play is often needed. Conditioning and strength development is critical in the 3-4. not only in the BIG, but everywhere.
You need studs up front and diverse players as backers. You also need strong cover corners......especially when the 3-4 aggressive style is used.
Down linemen need to have superior strength. And backers need to be fast enough to cover (even if it is for a short time), and strong enough to take on the type of back the Big Ten will throw at them eg Taylor and Barkley first come to mind (cripes, I saw a youtube clip of Barkley power cleaning nearly 500lbs....the man is a beast). The down linemen need to, at the very least, stop these type of backs from getting a full head of steam by the time they break the line of scrimmage. The backers need to be fast enough to get into position to lay a shoulder into these guys......arm tackles are going to make them look foolish.
The corners need to be able to cover tight without getting beat one on one....special guys to be sure.
Much has been said about exceptional OLB's and superior nose tackles, while true, these athletes cannot make up for bad players elsewhere.
OK, this is the info I got......but I'm still learning the finer points of this defense, and I'd love to be enlightened (or corrected) even more, so..........