The Positive Plane is an overpowering expanse of white light
Although D&D 5e's DMG has only limited information on these planes, the D&D 3e adventure module Bastion of Broken Souls (2002), which takes place on the Positive Energy Plane, describes it thus on pages 19-20:
This is a place of brilliant white. Pulsating brilliance bleaches out the spectrum and creates stark shadows. Anything more than 5 feat away becomes an indistinct blot of darkness moving against the white background—the blots nearby are certainly your companions. Distance is impossible to gauge within this brilliant environment.
An important facet of this knowledge reveals that living creatures do not last long before swelling and burst with excess life ... The Positive Energy Plane is like the heart of a star. It is a continual furnace of creation, a domain of brilliance beyond the ability of mortals to fully comprehend. Its very being wavers and ripples as new matter and energy is born.
The plane is a wide open undifferentiated expanse. The light is so blindingly bright that even tying a cloth blindfold over one's eyes, it is possible to see to a distance of 300 feet. Simply existing on the plane heals creatures, but this will soon overheal them to the point that they are annihilated by the energy.
The plane is not coexistent with the Ethereal Plane. Gravity is subjective; you fall down whichever way you currently imagine to be down, allowing adventurers to "fly" around with some practice.
Parts of the plane's energy have actually solidified into crystals which are invulnerable to any magic short of direct divine intervention. Entire structures of this form exist. One such location, a ten mile diameter hollow fractal crystal inhabited by numerous creatures, is described as follows:
Spans of crystal, miles long and hundreds of feet wide, pulse with a languide cadence of brilliance. The spans from the edges of what resembles, for want of a better comparison, a snowlake of titanic proportions. Only the edges are apparent; the bulk of the structure is lost in the glare. Specks of darkness congregate near the nexus of several crystal spans.
Interesting side-note: you can actually see a map of part of this structure in the finale of Buffy the Vampire Slayer, where they play a tabletop RPG and a map from this adventure module is used as a prop.
AD&D 2e Planescape sourcebook The Inner Planes (1998) describes that it's very difficult to actually get to the Positive Energy Plane. Many plane-shifting magic items are specifically built with a safeguard to stop the wearer traveling to the Positive Plane, since it's dangerous to travel there and there are very few good reasons to do so.
There's no air on the plane per se, but the life-giving healing energy of the plane sustains visitors without air, food or water.
The Negative Plane is an infinite black void
The Inner Planes describes the Negative Energy Plane as a limitless expanse of destructive, annihilation energy.
It is completely black, absolutely dark. Even sound does not travel far, and the sense of touch is itself dulled. Items conjured into the plane crumbles within one round, and living creatures suffer constant damage. Navigation is nearly impossible.
There is no air to breathe. The only creatures which live here are incorporeal undead, who thrive in the negative energy environment, though physical undead are soon annihilated by the plane's energy.