There is a lot that has gone into Madden 10 (don't forget to download the ultimate team too

. One of the big differences with 10 is the gang tackling and more of the stats are taken into consideration, including the players weight amongst a tonne of other skills.
If you have a back going through the middle and has exploited a whole and your safety or backer has pushed up to hit or tackle him then he may well break the tackle depending on how their skill sets are matched and also what position the back is in when the hit is made. I must admit this is the toughest Madden I have played so far but is much more realistic in so many areas and makes for close games. You madden iq will rise and fall in certain areas too and the AI adjusts to your performance to make those games much closer.