for me it's the case and the weight of it.
also stepped caps lock and 1.5 alt/ctrl
As far as I understand it, all KMAC's support either center stemmed or offcenter (stepped) capslocks. My KMAC has support of 87 key winkey layout.
The programming is terrible, but the case is very well made. That much CNC and hard anodyzing costs a bundle.
That said, it's not too expensive for a CNC metal case TKL keyboard.
I don't think it's that rare either, especially compared to other customs at the time. For a while the KMAC and the TYpeNow Solid were probably the only two metal cased keyboard you could get as a GH'er in America.
Ofcourse I am referring to cherry MX keyboards. Vintage metal keyboards have always been common.