I agree with everything N0-1_H3r3 said except one thing; I just cannot imagine the Webway having an atmosphere, being oxygenated and pressurised. At least not in the regular sense. Webways open up everywhere, including straight out into space. Wouldn't there be biomes? A slow buildup of growth, nevermind the animals that surely would slip into it on occasion? How would you ventilate it, or prevent pockets of carbon-dioxide in certain places, or concentrations of oxygen in others? Or whatever unholy thing that comes from the Eldar?
It doesn't obey the natural laws of reality, because it isn't part of reality.
A Gellar Field is a simple concept in its own right - a pocket of reality to ward a vessel from the corrosive un-substance of the Warp. The Webway is more than less than that - it is safe, in its own right, from the toxic influence of the raw Immaterium, yet it is also beyond the physical reality that most beings call home, and contains its own dangers and hazards to the unwary because of its extradimensional nature. The Eldar have long known many of the mysteries of the Webway, enough to be wary of its perils and watchful for them enough to navigate around them, but few know the Webway as the Harlequins of the Laughing God do, and they are loathe to speak even a fraction of its secrets.
In short… it's an extradimensional labyrinth realm, eternally shifting. The petty nuisances of the material universe need not apply.
Yeah, I might be overthinking it a bit, but I don't know, I just have a hard time seeing it. I think it feels more reasonable to believe that the armour worn by the people we see entering and exiting the webway on foot either have their own pressurisation, or that between whatever two points they travel between (such as the smaller webway gates on a planet) have structures within the webway to accommodate open, unprotected travel.
The Webway isn't purely a travel medium, though - the Eldar have had settlements within the Webway for tens of thousands of years at least, most of which have long since been fused with the Dark City of Commorragh and now sit beneath the light of stolen suns. Further, the smaller passages and gateways are actually far more common than the ones large enough to accomodate starships. Enough of the Webway is habitable to unprotected life that creatures actually inhabit it.
The heart of the Webway concept is closer to myth than science fiction. It is a place, but not one within physical reality. It is the paths that Eldritch beings use to step between the stars, their passage unseen and unnoticed by man. It is a gloaming place, bathed in perpetual half-light, home to tricksters, sages and bloodthirsty monsters alike. It is one of the wonders of the galaxy, the result of science so advanced that lesser creatures call it magic, and sorcery so complex that it may as well be the work of gods, so incomprehensible is its scale and the splendour of its construction.
On a related note, I like to keep more than a little mystery with the Eldar - they're ancient, cruel and inscrutable beings of alien intellect and an inhuman capacity for extremes of emotion, whose science seems as witchcraft to the men of the Imperium, and whose magic is subtle and potent in equal measure. As Phil Kelly and Jes Goodwin mentioned in their designer notes for the Dark Eldar revamp, the Dark Eldar are the nasty, nightmarish side of fairy stories… the impossible, spectral beings who appear in the night and spirit away friends and family members, who thirst for terror and sup upon agony. I like the concept, and I really feel that the Eldar as a whole benefit from being a little more "capricious fae beings" and a little less "pointy-eared aliens". They move in silence and secrecy between worlds along glittering paths that only they can walk, arrive unheralded and depart as suddenly, weave spells of misdirection and misfortune and their motives are seldom known to any but themselves.