Other desirable characteristics:
I don't actually know if these are important.
1) have some sort of error detection, like check digit, so user can tell if they have cut/pasted wrong. ISBN has check digit, as do the handles generated by the lsdi noid generation. (And OCR can tell if the identifier is wrong. why should we care about OCR? See cell phone scenario below)
2) have a fixed length, and limited syntax. Why? If we pick a type of identifier with a large identifier space, but an extremely rigid syntax, the identifier is then more easily found in html pages, and manipulated by such things as greasemonkey, or libX. In addition, if we are dealing with an identifier that has been printed, and then needs to be recognized by OCR (think about a cell phone camera grabbing the doi's from a printed pdf, and bringing them up on your laptop screen), the job is much easier (I mean, actually possible) if the identifiers implement a very rigid syntax. arxiv identifers, namely, arxiv:xxxx , are really quite good in this respect, I think.
Keeping these things in mind, something to think about is fixed size handle.
Another system along these lines is the UUID system.
http://en.wikipedia.org/wiki/UUID
A UUID is a 16-byte (128-bit) number. The number of theoretically possible UUIDs is therefore 216*8 = 2128 = 25616 or about 3.4 × 1038. This means that 1 trillion UUIDs would have to be created every nanosecond for 10 billion years to exhaust the number of UUIDs.
In its canonical form, a UUID consists of 32 hexadecimal digits, displayed in 5 groups separated by hyphens, in the form 8-4-4-4-12 for a total of 36 characters.
550e8400-e29b-41d4-a716-446655440000A UUID may also be used with a specific identifier intentionally used repeatedly to identify the same thing in different contexts. For example, in Microsoft's Component Object Model, every component must implement the IUnknown interface, which is done by creating a UUID representing IUnknown. In all cases wherever IUnknown is used, whether it is being used by a process trying to access the IUnknown interface in a component, or by a component implementing the IUnknown interface, it is always referenced by the same identifier: 00000000-0000-0000-C000-000000000046.