No optical drive?
I'm a videographer whose clients almost always request a DVD of a video.
I also have a large collection of CDs I'd like to import into iTunes.
No optical drive will make both these points a problem.
Apple gets too far ahead of itself for the sake of a slim design.
How often do you burn DVDs or rip CDs on the go? The optical drive isn't going away, it's just going external. It might be marginally less convenient to take with you, but for all the times when you don't need it on the go, you can save some weight.