The biggest loss (and it's a biggie) is the double-high escape key. The rest is a smaller set of biases from a better-designed keyboard over 7 rows to a less well designed one in 6.

I've got a W520 (old keyboard) and my wife's got an X1 (not carbon). There's enough of a difference in the keyboards that I'm hoping they bring back the old one as an option. The keyswitches in both are pretty good, but the layout's actually a big deal for me.