Supposedly, the sound is going to be dramatically improved, from the commentators, to the crowd noises, to the quarterback audibles. The passing game has been refined with better ball placement of your throw, different throwing styles (a new shovel pass should be interesteing) and over 430 catching animations. There should also be better and truer AI. Defensive players will react to the ball if they're looking in that direction, but won't psychically know where to run to.
For all the known updates check out http://en.wikipedia.org/wiki/Madden_NFL_13
Also, most importantly, Tebowing is included! http://bleacherreport.com/articles/1190386-madden-13-tebowing-is-a-hilarious-addition-to-the-classic-video-game (Or as my mom calls it "Doing The Tebow.")
Thats awesome. Getting a modern Madden will also allow you to play online for a year or so. Is that part of the reason you will be getting it?
Actually I totally forgot about that part (since by the time I get an EA game usually it's offline only) lol. Rock on!