IBM Model F 122, ANSI Modded.
The size. This thing is so big that I get shoulder pain if I try to use my mouse to the right like other keyboards. Chop off the numberpad, slim the bezels and this thing would be perfect.
Such a beast almost exists, too... the Unsaver, as well as the Kishsaver's 77-key sibling. Those are rather hard to find though, and still have the thick bezels.