The point about HDMI and the 10w power consumption and the lack of a spare power brick is that this device is being positioned as a living room device, not a desktop machine. Add bluetooth keyboard and mouse, FrontRow, optionally an Apple Remote, and a big TV set and you've got: DVD player, Apple TV substitute for streaming movies/TV via iTunes, household iTunes streaming hub (streaming video to iPads in other rooms, streaming music to Airport Express-driven active speakers in other rooms -- controlled via Remote.app running on your iPhone), and if you enable internet sharing and use Cat5e to your DSL/cable modem you've got a servicable wifi hotspot as well.
This isn't being positioned as a desktop, it's being positioned as a replacement for the Apple TV box.
And for those of us who're into getting stuff done rather than sacking out in front of the telly, there's th slightly more expensive model with no DVD, more hard disk (RAID 0 or RAID 1, anyone: OS/X has software support for RAID), and a server-grade OS.