The thing about a 3-4 defense, is that you can bring a 4th defender [or more] from other places, but you DON'T HAVE TO bring a 4th defender. Yes, with zone blitzing and what the Crimson Tide and Steelers do from various packages is one thing, but there is no rule stating you have to bring a minimum of 4 defenders per play.
The nose tackle is supposed to be so big and huge, that he can be two defensive tackles in one, leaving only thee on the line, with 4 linebackers and 4 defensive backs in a normal 3-4 set.
I don't ever see us going to a 3-4 defense under Pelini. He is always worked with a 4-3 and always talks about playing a two gap system and how much he loves his gap system. The only thing that would resemble a 3-4 defense would be a "Spinner" package, but that in itself is not a 3-4 base defense, it is more designed to stop the pass and confuse spread offenses.
Most of the inconsistencies on defense last year stemmed from inexperience, in certain spots we were too slow, not a lot of talent on the field [at the time, although they nicely did progress in certain areas], to injuries, and to guys not communicating and not being a cohesive unit. We lost a ton of talent from 2010, especially with the injuries to Crick and Dennard, that we looked like a brand new green and young defense. It was a very average defense last year.
Pelini brings pressure when it's needed, but if you bring pressure you are usually asking for man coverage with no safety help behind you, unless you are zone blitzing, and we may not have had the experience or talent last year to do it consistently. He did state this year he wants to "get back to the style of defense he's known for playing" so either that is blitzing a little bit more or just being better overall, let's just hope it makes the defense a world of difference from last year.
I still believe that 2011 is like 2008 for Pelini. He was getting adjusted to new conferences offense just like in 2008 when he became a head coach. Hopefully we see a great leap in the defense this year for 2012 that we saw in 2009.