Which are the "axis" of representations of a REST resource

Which are the "axis" of representations of a REST resource

Given an "abstract" resource, I'm looking for all different ways it could
be actually represented.
So far, I can name :
Content-Type (eg. XML, JSON, YAML for a semi-structured document)
Language (eg. fr-FR, en-US, ...)
Version (given by Accept-Datetime header ?)
Could you name others ?
I have a dilemna about authentication : is /home a valid rest resource, or
should it be /~me/home ?
Additionnaly, could you say which HTTP header is suited for each
representation "axis"