i'm also a big fan of '04. i skipped '05 but went ahead and got '06. so here are my comparisons based on 3 games and poking around the other features over a 5 hour period.
- the ability to send in different personnel packages is a big plus. now i can line up steve breaston at QB to run the option just like in real life. if the defense's #2 corner gets hurt, i can immediately switch my two WRs with one push of the button (to get my #1 WR matched up against now their #3 corner) and take advantage of the mismatch. i know you could do this manually in '04, but it's much much quicker to do in '06.
-"impact players" is not that dramatic of a feature. if you know your roster well enough, you know who your impact players are without the white shadow under them. if you scout the other team's roster just before you play them (which i do), you know who the opponent's impact players are.
-ball physics and DB animations appear to be more realistic. in '04, the ball could just bounce around like a pinball among a group of players. i haven't seen this in my first day. and in '04, half of your pass breakups happened when your DB gets in the perfect position, gets two hands directly on the ball only to have the ball bounce of his brick hands, and corso ridiculing the DB. now, the pass break-up animation has the DB reaching his hand out at the last moment to make the play. it's a complete 180-turnaround in emotion. instead of cursing at your DB for dropping an easy pick, you celebrate because your DB made an awesome play.
-as you turning the corner when running around a defender in '04, the defender will turn his torso toward you and run with you an inch away from you, but does not make any attempt to tackle. in '06, the defender will make the dive in attempt to tackle from behind. this is a welcome change.
-i cheesed the CPU by recuiting speedy QBs in '04 and could get away with running outside the pocket and having a field day. in '06, the chances of you succeeding with this tactic is dramatically decreased. the DE isn't glued to your T anymore.
-if this matters at all, you can get away with more celebration.
-run blocking, especially downfield, appears to be smarter. (thank goodness, i had WRs in '04 blocking the wrong defenders)
-the CPU seems to make adjustments to your defensive tendencies. on one play, i was in a 4-3 and shifted my strongside DT WAY over to the weakside leaving a hole in the defensive line. on the following play, the CPU ran the ball exactly to that hole. coincidence? i'll be paying attention.
-in '04 you could call audibles and hot routes as soon as you break huddle. in '06, you cannot call those until the QB is set in position. this has been siginficant because there is a lot less time to make these adjustments. in '04, i could call 3-4 audibles until i was happy. in '06, you just don't have this much time. this is for the better.
-there is apparently coverage audibles for DBs and spy audibles for LBs. so if you are a strategy geek like me, this makes the game deeper.
that's it for now. if i notice anything else major, i'll come back and post in this thread.