I thought this was an interesting discussion... as it would give quite a bit more insight into what's going on in those frequency response and CSD graphs that we have access to, and it may give way for more research later.

Personally, I hear frequency ranges like this:

<20Hz: felt as puffs of air
20-60Hz: rumble, theater/battlefield atmosphere stuffs
60-150Hz: impact
150-300Hz: bloat when resonant or distorted
300-500Hz: warmth in vocal, or some may call it "bass bleed"
500-1KHz: cuppy, vocal emphasis, boosting this range will most likely cause a forward/in-your-face sound
1-2KHz: what some refer to as "upper midrange". Audio Technica headphones in the past typically boost this range quite hard core (can be seen on FR graphs)
2-4KHz: "shouty", as some would say. Having a healthy amount of this range will give "texture" to vocals, but too much and it's harsh/annoying
4-6KHz: "edgy/sharp", I think this range specifically boost "definition" and "resolution" per se. But too much of it can be grating (a la HD800)
6-8KHz: sibilance... plain and simple.
8-10KHz: peakiness, sparkle, some call this "high frequency extension", but I hear it more as "limit of my ability to withstand high frequency sounds"
>10KHz: just gets more and more piercing until it dies off. Some call this region "air", but I don't think so. I think the misconception stems from the fact that when people force an EQ for these frequencies, their headphones sound more "open". Personally, I think the phenomenon is more closely related to how DSP (either in software or hardware) is shaping the impulse in order to match the EQ settings, and as a result, the impulse decays faster than usual, thus giving this impression of "air". This is just a guess though, from simulating square wave and looking at their forms when EQ is applied. I'm not sure this is the case in reality.

In my opinions, what many refer to as "air" is simply a faster, or more "natural" decay of mid to high frequencies. This can be achieved pretty easily by open-back headphones because they have very little enclosure resonance as opposed to closed-back headphones. Most closed-back headphones are very poorly damped, and this prevents them from being "airy".

On the other hand, being "airy" alone doesn't necessarily mean a headphone will have great soundstage. I think "air" aids in accurate reproduction of sound imaging and "separation", but "soundstage" is an entirely different ballpark that probably has more to do with the acoustic space in between the ears and the diaphragms. From what I can tell so far: distance from ear to diaphragm determines distance to sound source, and thus aids in creating "depth" and "width", and angle of the driver/diaphragm relative to the listener's ear would determine the "shape" of the soundstage. But I haven't been able to pinpoint exactly what would create a good "height".

Anyway, please discuss, and be merciless in your refutation/agreement of my opinions.