MacBook Air missing more than an optical drive
Ethernet, FireWire, user-replaceable-battery not there.
The new MacBook Air is svelte-as-can-be, but it's also missing some key, traditional Mac functionality that might leave some users disappointed or in a lurch. First off, the battery is apparently not user-replaceable. This means you can't swap out batteries to extend operating life, and you'll likely need to seek authorized service to get the battery replaced when it inevitably loses capacity or if it fails altogether.
Since the MacBook Air lacks an optical drive, you can't boot from an inserted DVD like the Mac OS X Leopard install disc unless you purchase the optional, $100 external SuperDrive. It's not yet clear whether the MacBook Air can boot from an optical drive in another Mac via the "Remote Disc" function, but we doubt it. Further, if you can't boot from an installer disc, how will you be able to install the next major iteration of Mac OS X? Traditionally, Mac OS X installers have required the system to boot from a disc.
There's no built-in Ethernet port. This means you can't (apparently) network-boot unless you purchase the $30 USB-to-Ethernet adapter.
The MacBook Air also lacks a FireWire port. This means you can't use FireWire target disk mode -- an invaluable troubleshooting tool and data transfer tool. In addition, some peripherals (including a host of digital video cameras) that are FireWire-only will be left out in the cold.
Finally, you can't upgrade the MacBook Air's RAM (you're stuck with 2GB, the default) and there is no audio input port, though a built-in microphone is included.