Ratings...ratings...ratings...
I don't care how much you fiddle with sliders...coach mode cannot be made "realistic" unless you change ratings for all the players.
However, QBs are the ones which really make the most difference. Their Pass Accurracy and Pass Read Coverage ratings really need to between 90 and 127 (large attribute ratings in Finn's editor). However, QBs are just the start...every position needs "tweaking" for coach mode to play correctly, generate realistic stats, and to just look "right" for the on field gameplay.
I don't have the time to detail all the edits that are required right now, but will try to post something later or tomorrow. In the meantime, take a look at the thread started by Daflyboys where he details his take on adjusting ratings. It's very close to what I subscribe to as well.
Daflyboys' Tweaking Guidelines for '07
C