Some good thoughts here, everyone. I pretty much agree with everything that's been said, which shouldn't be too surprising.
I guess what this grows out of is a feeling that strictly location/tool -based Contexts aren't helping me as much as I'd like. I only really have two or three of them, which doesn't give my Next Actions lists enough structure to really be helpful.
So we've got the basic Four-Criteria Model:
2. Time available
3. Energy available
Merging together 1, 2, and 3 gives me a sort of "super-Context" that seems useful enough. Energy Available is sort of a tricky one -- I think of it in terms of not only how much energy, but also what sort of energy.
Anyway, it seems to be working out alright so far; I'll try to provide an update later on if I have any new insights.