I received a request to try and adequately describe what subspace is. I'm not sure if I can do this or not, but I'm going to give it a try. While there may be some medical connection of subspace to the production of certain brain chemicals and those brain chemicals may have some addictive qualities, I'm not well-versed enough in what those may be so I'm going to steer clear of it for now and it probably isn't really relevant to this post anyways.
Subspace reduces someone to a child-like state of focus. In this state the Domme becomes the sub's whole world in the same way that a young child might feel like his parents are the whole world.
He will define himself in terms of her. Her behavior towards him and her reactions to his actions will determine how he feels about himself. The morality of this world is determined by what brings her pleasure and displeasure.
The feelings are difficult to describe but it's like your soul is overwhelmed by your love for her and the only thing that matters is what she desires. It is a total surrender to her will. In this state the world makes total sense.
While subspace and arousal are heavily linked, they are not the same thing. e.g. Rubbing a man's cock will get him aroused but it doesn't mean he's in subspace. Once subspace is introduced, arousal and subspace can often feed one another in a loop with deeper subspace leading to greater arousal and greater arousal leading to deeper subspace.
Subspace has varying degrees at any time that can be "shallow" or "deep" depending upon the situation. It's a lot like being drunk.
Early levels: Willing surrender (equivalent to being tipsy).
Middle levels: Desperation to please (equivalent to being drunk).
Deeper levels: Loss of self and independence (equivalent to being wasted).
No subspace at all is the equivalent of being sober.
**Note: I am not comparing subspace to intoxication, just that the intensity of subspace varies from mild to extreme in a similar way and it is usually a process of moving deeper or more shallow.
Environmental variables have a lot of control over subspace. These are often symbolic or developed through association but they can be used to trigger subspace or deepen it. A collar is an easy example of this. If a Domme orders the sub to get his collar it may trigger an immediate drop into subspace or push him into deeper levels of subspace. Rules and fear of punishment can also trigger subspace even if the Domme isn't immediately present. With certain things in place, it's possible to create a subspace loop without that will perpetuate itself within the sub.
Depth of subspace often affects what a sub is willing to endure and enjoy. If deeper levels of subspace are reached, the sub is more likely to willingly submit to more extreme/intense activities. He may hate something physically but love it emotionally because of subspace. If he is in an earlier stage of subspace he may (want to) reject an activity outright but when in deeper stages of subspace it may turn him on. The deeper the subspace, the narrower his vision will become and at its deepest stages, he can only see her.
Orgasms can affect the difficulty of achieving subspace. The more recent the orgasm, the more difficult it can be to return to subspace and reach its deeper levels. This is why many Dommes create a post-orgasm ritual or activity to eliminate or reduce this down time.
Subspace can be addictive. A sub may not only crave to be in subspace, but as he progresses, he will inevitably desire ever deepening levels of subspace. Ideally this progression will mirror the Domme's desire to escalate their D/s relationship to greater levels but this is not always the case.
Overall, I guess subspace could be described as the intensity of emotional surrender a sub feels towards his Domme.
And yes... this post was woefully inadequate at capturing just what makes the feeling of subspace so "magical."