For me, verifiability is the first thing I think of as it is a kind of meta-property of bitcoin in addition to being a feature or property of bitcoin. Meaning, I can verify all the properties that Lopp enumerates by reading the code and understanding the system design in addition to more tangible things like block difficulty/order and transaction validity.

Lopp addresses this in the open source section and touches on the asymmetry of verifying vs computing (aka proving), but I don't think bitcoin would be bitcoin without this which is why it's "key" to me.