Something's got me thinking about spirits and how they were sort of glossed over in Planescape.
For the most part back in 2e, they just seem to be another sort of monster. Granted some of the monsters described as spirits are creatures with an intrinsic connection to a plane, and possible explanations for spells like Speak with Rocks and so on.
In 3e they evolved the concept of spirit a little more. They not only had a bunch of the explanations in 2e, but they released a few Shaman classes and started implying the Druid talked to similar things. Said that many spirits were in fact Rank 0 Deities that were numerous, and introduced the alternate cosmology plane the Spirit World. I know that generally the idea of alternate cosmology planes are heresy here (like the Shadow Plane being a full plane), but I felt it was an Spirit World brought an important dimension on the way things were.
4e went further with the primal power source, and they've elaborated a bit more in primal power describing some well-known spirits like the Primal Beast, World Serpent and World Tree. They describe the Spirit World as an unseen part of the world, sort of a state of being that only those in the right state of mind can perceive it.
The problem with those explanations of the Spirit World is that I feel they are too prime centric. I wrote some thoughts on this when I described the primal power source in Planescape. For one thing I feel that every plane has it's own versions of spirits, some are very much like the primal spirits of the material plane. Other spirits are in fact the Powers, and things like celestials and fiends. Though the later two less so, since they've become more like beings in having generally corporeal and fixed forms.
So my idea is that the Spirit World and Spiritual Reality is a state of being that exists in every plane. The interactions between spirit and physical reality vary on each plane. Again there's a case of most mortals not being able to perceive spirits, even the most canny planewalker unless they happen to be of a class (or know the right spell/ritual) that does such things. But the difference is that many spirits on the planes do make themselves seen and known far more than the spirits of the prime.
I actually liked the concept of Levels of Reality that was discussed in the module Doors to the Unknown and the sourcebook Ravenloft: the Nightmare Lands. I would put spiritual reality on the 4th level, even if there might be some issues reconciling hyper reality with spiritual reality.
Did you ever see the Shaman book for 2nd edition? It went into great detail about what a spirit was; On Hallowed Ground seemed to reference it (it didn't explicitly, but that book's discussions about the origins of certain pantheons was very compatible with it). It's fairly close to the 4th edition definition.
Basically, according to Shaman, the Spirit World doesn't exist. Spirits are made from powerful emotions and beliefs, and they may seem to appear or disappear, but there's no actual other plane or level of reality that they're going to. They are simply manifested or they're not. The spirits and shamans, however, all believe it exists.
The history of the Shaman book is interesting. It was originally developed by Mayfair Games as part of their RoleAids line of AD&D-compatible products. After TSR sued that line out of existence, it was one of two unpublished RoleAids products (the other was Chronomancer) that TSR decided to rewrite and release itself.