I used to be nothing but a 4-3 person but recently changed to the 3-4 in '10 because of the ability to more easily stop the pass (since this game is so completely pass driven) and I just seem to play with teams where the 3-4 better fits my personnel.

I never have any problems with the 4-3 stopping outside runs, but with the 4-3, I rarely play cover 3 zone coverage so my corners are almost always at the line of scrimmage either in man or cover 2. I also regularly more my line toward the strong side of the formation and then my linebackers based on the play (blitzing with the ROLB or LOLB then I move them toward that side or if it's an overload then I'll overload the weak side most often while slanting the line toward the strong side or out). I also regularly move my line to the strong side of the formation (be that the unbalanced weakside or the strong side with the TE if it's just normal I-formation) and then spread my linebackers out (especially if I'm playing straight Cover 2 or to change up my blitz looks).

I'm not one to use overload blitzes as effectively as some other players, but even just the threat of bringing an overload usually causes the other team to think about it and change their play in some way (be that keeping the HB/FB/TE to block or giving himself a man beater go to route that I can then key in on) so it's a win/win for me.

Mainly my only issue with it is the inability to play good Cover 3 defense and still get good pressure and and variations of Cover 3 is what always makes it the hardest on human players to throw the ball.