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.
Also, from what I've read the underlying key mechanism is the same. The caps have been made larger and they're curved and thoughtfully shaped.
I played with the new ThinkPad keyboard at a retailer and I think its better.