All 4, but mostly the layout and shape. There is space on the keyboard deck, yet all sorts of buttons are hidden under fn, and for people who use pgup/dn / home

All 4, but mostly the layout and shape. There is space on the keyboard deck, yet all sorts of buttons are hidden under fn, and for people who use pgup/dn / home/end, it is a big change. Also, I think the thinklight is vastly superior to the backlit keyboard for illuminating the whole area around it.