To try and establish a set of rules for UAV airworthiness is akin to writing a set of rules for everything from a paper dart to an Airbus A380.
I'd personally advocate using the appropriate manned design code (S, VLA, 22, 23, 25) and then removing or modifying what is inappropriate to a UAV. There's a whole industry that knows how to comply with these codes, so then the first major difficulty is what to remove - some bits are easy: you can drastically cut undercarriage requirements, remove most handling qualities requirements, possibly reduce Nz limits to what's permitted by the FCS. This approach is probably inevitable, although I'm not aware of much standardisation yet.
The second major difficulty, and the biggie, is then the control system - what do you require in terms of autonomy.v.direct control, reversionary logic (i.e. what does it do if the world stops talking to it), how does it sense and avoid other traffic. Tens of millions of (insert currency of your choice) are being spent on solving this one, which really is the holy grail of UAV utility.
The philosophy of all of this will remain the same however - ensuring no greater risk to people and vehicles outside the UAV/UAS than would be the case if it were certified as manned and flown by a qualified professional pilot.
G