Which is surprising, because the Steelers have been using it since the mid-90s and have had one of the top defenses in almost every year...
But, I digress, anywho, there are two things you need to switch to the 34:
1.) The personnel to run it. Mainly, you need outside linebackers. You can't plug in a OLB from a 43 into a 34, cause they don't have the rushing skills. You can't plug in a DE acuse they don't have the coverage skills. YOu actually need to draft and train OLB's in the 34. Most coaches and scouts predict it takes a good 3 years (much like a WR) to train a small, speedy DE into the mold of a 34 OLB.
2.) The coach and coordinator to implement it. Its one thing to just drop a lineman and add a LB and try to send in more blitzing... its another to actually get it to work. Most DC in the NFL are brought up in strong 43 and Cover-2 defensive schemes, not the 34. The 34 requires a LOT of imagination and cunning. Its a weak package straight up, but in the hands of a smart schemer, it can confuse the hell out of opposing OC's and wreak havok causing turnovers and sacks galore. The only coaches that I know for sure that can run the 34: Romeo Crennel, Bill Cowher, Don Capers, Jim Haslette, and Dick LeBeau. All are head coaches or have been head coaches (in LeBeau's case), now.
We are seeing quite a few teams making the switch, but all but one will probably be successful in it:
1.) San Francisco: Mike Nolan likes running the 34. He switched Baltimore over to it. Baltimore was better in the 43. He coaches it decently with great personnel... the SF experiment will probably fail (though there is a chance since its in the rebuilding phase).
2.) New England: Well, they've always had the 34 in the mix, but they are missing their coach so lets reevaluate. I'm thinking Romeo was the brilliance behind their 34 packages (don't give me the Brueschi factor, he's replaced by the very capable, and been in the 34 scheme before Chad Brown), because they are resorting to hardly ever using it anymore.
3.) Cleveland: Here is the success story. Its obvious Cleveland doesn't have the personnel... yet, but Crennel was the mastermind behind the NE 34 defense, and is already showing success in it in Cleveland (held Manning to no touchdowns, etc...). After the reconstruction is complete, this will be a fine defensive team.
4.) Dallas: Why is the big tuna still bothering? Its obvious his defense stinks to high hell. The 34 experiement already failed, now cut your loses and go back to the 43.
I know there are other teams experimenting with the 34 (Miami, Oakland), but I have a feeling all will abandon it by next year. Just don't have the 2 poitns and aren't willing to undergo the time it takes to convert...