Ok so it's the same as gentoo from what you say (this wasn't always true, or it didn't always work at least) except the user can't modify it? It sure would be nice to give the user access to this list and even to be able to edit a list and use the edited version to configure a new install, just as one could with the world file in gentoo. And at the same time, might as well call it a world file and a link it to /var/lib/portage so portage has access to it also.
update:.. ok so having recently installed a new system, just checking the world file there... sure enough, it no longer includes everything(yay!). In fact it appears it only includes things I asked for after the disk install and probably no description of things from the original install. (Now I'll search for something equivalent to a --oneshot option) This is certainly useful even if only to be able to write a script later to grab things I particularly wanted, but it seems to be only differential to the default install choices which can change with new releases/selections. Of course in a default install the admin didn't ask for any packages explicitly so there's no perfect way to describe that. Still some reasonable starting set of top level entries and a clear idea/way of how one should/can use that to rebuild or re-synchronize a system sure would be nice.
Maybe the only thing missing is my own understanding of how I am able to do these types of actions now. Example with portage just drop in a world file from anywhere you want and do
emerge -NuvaD world
all set.. completely redefined system (and I've done this, it works).
I guess now I can use the methods suggested (including my own) to build that reasonable starting point of a world file along with the entries that exist now. And then use portage itself to do the depclean on an edited one for a new system, it just seems a little hacked/ unsupported/ not entirely clear how it will interact with entropy moving forward.