Thunderbolt
SD expansion
256GB SSD is plenty - can upgrade to 480 with an aftermarket SSD
Fast processor - yes, it is fast.
Enough RAM to run pretty much any productivity app you need
Examples of what I run:
CAD software - DraftSight/AutoCAD
VMWare Fusion with 1 Win7 VM, 1GB RAM dedicated 24/7 to it, running Visual Studio Pro in unity mode
Xcode 4.3
Office
And all the small stuff
Macjournal
Textwrangler
itunes
Preview (with PDF manuals)
OneNote in VM unity
Active Directory Users & Computers in VM unity
Hyper-V manager in VM unity
All of this runs, all the time on my MBA. No slowdowns. My daily work is stored on the laptop, my backups on a USB 1TB drive, my music in the cloud, and I frequently move large files over the network.
For network equipment configuration I have USB adapters for Ethernet and RS-232.