Performance and form.
I have a powerbook now because the ibook was underpowered (im a new yorker, i hate waiting), but the new MBP is overpowered for my purposes (not pro).
The next redesigned apple laptop might be on my lap some time, but as my sig says, i HATE old designs. My current powerbook is almost fast enough (i play alot of games) so i don't need 4x faster. My next laptop will be based off of software and design, its hard to go downhill from my computer.