a) Not visible or accessible from outside (so the coherency/snooping/ordering issues go away)
b) Not subject to address translation (so no synonyms, no waiting for
c) Word-addressed (preferably), or at least with very clear rules of who can overlap what, and when, that can be decided purely from the addresses,
d) With no support for dynamic indexing (so you can rename/disambiguate easily),
f) with no privilege architecture for the high-traffic accesses (to get that headache away),
g) with no possibility of any missing pages or other faults occuring from accessing said memory,
...but every single one of these gets you away from "just like regular memory".