The grip is textured differently, and it's a bit longer to accomodate a 16 round mag in .40 caliber. It also has upgraded sights, and supposedly and upgraded trigger.
I actually have a fair amount of respect for the XD, but I've handled the XDm and was unimpressed. The longer grip would be harder to conceal, the sights would be harder to hook on the belt to cycle one handed, I couldn't tell any difference in trigger pull or reset. It wasn't that there was anything wrong with it, but I guess my idea of improvements is different than Springfield's.
I have been a fan of the xd's for awhile. (I have a 9mm HS-2000) Had it since before I had Glocks. I really like the XD's, but this XDm makes me wanna puke. They pushed it into the market too hard and to me they came across as wanting it to be portrayed as "new" and "cool". Not what a quality firearm is supposed to be about IMO. I really like the added (now normal) capacity in .40, but the barrel length is kinda too long to be short and too short to be long. Just hard to beat the options of a Glock with the 27, 32, 22, & 35. And 24's can still be found. You can change the backstrap on a XDm, but your stuck with the same sight radius and barrel length. The XD was never a player in the .40 field IMO. 12 rounds in a gun the size of a Glock 22 is just not even in the same ballpark. They did pretty good with the 9's and .45's though.
I've handled the XDm when it hit the shelves. In my personal opinion I think its okay. Never shot one. I wasn't very impressed with the trigger pull (still), thought the grip was a little long and the overall visual asthetics was just to busy. On a positive note, I really like the sights, the grip safety and all the metal components the XD uses such as the trigger, etc.
Anyway thats my opinion and its subject to change at anytime.