It will drastically change QB play.. Duo QB's will no longer be effective..if one comes in the middle of the game he will be "cold" and take an attribute penalty.
I heard there are also changes to m2m coverage, because WR's were winning 90% of the 1v1 battles or something like that.
Check out the MFN-1 beta chat thread, lots of input and JDB has a list of all the changes. Setherick and Raymattison are (or were) heavily involved in the testing. i was there not too long ago but had to drop my team. i got to see the new code and it was a disaster.