The following concepts are taken from 4e (sorry) due to the fact that they seem to have given a little more thought to things in this edition. They don't differ greatly from 3.5e and there are no rules differences, just fluff.
The Astral Sea
The Astral Sea is the plane of the gods and demigods, a model of perfection and order. It includes Syriana, Irian, and Daanvi.
Syrania
Syrania, the Azure Sky, is the perfect paradise that most people would want to retire to in the afterlife, but currently the only known residents of Syrania are angels. With cities floating in a crystalline blue sky thanks to the light gravity on the plane each one goverened by a powerful Solar, it’s hard to imagine a more perfect existance. Sharn the city of towers is built in a Syrania manifest zone providing the city the ability to support the monstrous towers of the skyscraping metropolis. Clear blue cloudless skies and deep midnight blue skies mark a coterminous Syrania. When Syrania is remote the skies are slate gray or white and the nights are lightless and black. Sharn has a significant manifest zone of Syriana.
Irian
Irian, the Eternal Day, is a plane of positive energy. Some might equivocate this to paradise. A brilliant white sun hangs in a beautiful blue sky while rivers of liquid glass flow down from mountains of quartz. The plane is suffused with positive energy not enough to harm life, but enough to sustain unusual forms of life, though not neccesarily good aligned creatures. Just like its opposite, Mabar, the energies of this plane work in opposition to the natural decay and birth of life. In the short period of time when Irian is coterminous with the Material Plane life blossoms. Days are brighter, colors are vibrant and sensations are more intense. Positive energy spells are enhanced in these times, however the manifestations occur only for the hour surrounding noon. The plane is coterminous for ten days in Eyre. When Irian is remote the opposite takes effect. Night becomes darker, numbness hangs in the air and spells with positive energy are less powerful. These manifestations occur during the autumn month of Sypheros for ten days. An almost permanent manifest zone of Irian exists in Aerenal.
Daanvi
Daanvi, the Perfect Order, is the manifestation of law in the Planes of Eberron, and its opposite is Shavarath. All things of Daanvi live in perfect harmony born of order. On the plane all spells with the lawful descriptor are automatically maximized and enlarged, while chaotic spells are impeded. Though there are no founded links between changes in Eberron during coterminous and remote periods of Daanvi, many claim that centuries in which Daanvi are coterminous are historically more stable while periods when it is remote are historically unstable.
Dal Quor
The Realm of Dreams. Dal Quor is a unique plane in that it is shaped by the dreams of the inhabitants of the Material Plane. Every tree that you dream of sprouts in Dal Quor, but every nightmare monster lives there as well. Because of these principles, the plane is an almost constantly changing place, where no terrain is permanent. The inhabitants of the plane are mostly the quori, coalesced around the Dreaming Dark.
The plane came into existence when the first being slumbered and allowed the energy of their mind to escape the Material Plane. As the total number of dreaming creatures has increased, the plane’s power and size has increased.Most planes manifest themselves physically on the Material Plane, but because of Quori Giant War, Dal Quor has been forceably disconnected, and any cotermination is probably millenia away.
The Elemental Chaos
The Elemental Chaos is the plane of the primordials, and contains Shavarath, Risia, Kythri, and Fernia.
Shavarath
Shavarath, the Battleground, is a region ever gripped in the throes of war as three groups vie for control of the plane, a host of celestial archons, an infernal reginment of devils and a swarming horde of demons. Forever in a rough balance of power each of the three factions lay siege to one anothers fortresses only to lose their own holdings on other fortresses in the process. Beyond these fortresses the plain stretches infinitely, with low hills providing the only terrain feature. The plane is aligned by territory. Those areas held by the archons are mildly good and lawfully aligned while those areas held by devils are mildly evil and lawfully aligned and that held by the demons is mildly chaotic and evil aligned. Magic spells that mimic, create or enhance weapons as well as spells that excite hostile emotions are extended, while charms and pacifisms are impeded. Whirling blades travel the plane like flocks of birds seeking out non-native inhabitants and attacking them. In times of conflict, when Shavarath draws near to the Material Plane, the effects spill over into it. Magic spells are enhanced or impeded as they are on the Battleground and reports have even come of flocks of whirling blades seen on the battlefields. No known effect manifests when Shavarath is remote.
Risia
Risia, the Plain of Ice, is a plane that is layer upon layer of snow and ice with a constant blizzard raining down new layers of sleet snow and ice from a dark grey clouded sky. The very atmosphere freezes the lungs and steals away the breath. The average temperature hovers around 0 degrees. Most of the effects of Risia’s coterminous phases are unfelt by the general populace of Khorvaire since these effects mostly take place in the Frostfell and the Everice regions of Eberron. These areas grow more intesely cold and temporary conduits to Risia open up in these areas of intense cold allowing natives of the plane to slip into the Material Plane. Mostly, this is unfelt since few of the civilized races exist in these extreme environments. Similarly these remote areas experience a decrease in temperature around times that the plane is remote, though the change in temperature is only faintly noticeable.
Kythri
Kythri, the Churning Chaos, is the opposite of Daanvi. Highly morphic the land is a roiling mass of land, water, air, fire, magma, steam, and all other manner of elemental forces. The plane is in a constant state of unrest though some can keep a location from morphing by strength of will, if only for a short period of time. The gravity changes randomly in random directions, and spells with the chaotic descriptor are automatically enlarged and maximized while spells of the lawful type are impeded. Kythri does not follow a set planar orbit but rather lurches through the Astral Plane at random intervals sometimes hurtling through space, sometimes slowing to a crawl. Because of this feature Kythri might be coterminous with the Material Plane for a day or a century and might be remote for only a few months before jumping back into touch with the Material Plane. Popular superstition link coterminous periods with eras of unrest and war. Many scholars believe Kythri was coterminous during the first decades of the Last War but no firm evidence backs these claims. As of yet, there does not appear to be any effect on Eberron when Kythri is remote.
Fernia
Fernia, the Sea of Fire, is a continually burning landscape. Plates of compressed ash and debris float atop oceans of liquid flame as mountains ooze with molten lava. When Fernia is coterminous to the Material Plane areas of extreme heat become even more radiant and entering pools of lava may whisk one away to Fernia. Likewise inhabitants of Fernia can slip into the Material Plane and magic cast in these areas with the fire descriptor is automatically enlarged and empowered. When Fernia is remote, areas of extreme heat are reduced in severity. Spells with the fire descriptor are more difficult to cast, except near hot-spots.
Feywild
The Feywild, a wild reflection of the Material Plane, is the plane of primal spirits. It contains Thelanis and Lamannia.
Thelanis
Thelanis, the Faerie Court, is similar to both Eberron and Lamannia in that it bears untamed expanses of rugged forests and crystal-clear waters dotted with small settlements. Arcane spells are enhanced on Thelanis and time flows differently. Every day in Thelanis equals a week on the Material Plane, this time catches up to any person traveling back to the Material Plane. If one spends too much time on Thelanis moving back to Eberron might suddenly kill the person of old age. When Thelanis is coterminous to Eberron certain magical areas appear such as faerie rings and faerie mounds often accompanied by mysterious lights. These areas are conduits between Thelanis and the Material Plane and inhabitants of either plane can find themselves easily transported back and forth. Arcane spells are extended when cast within 300 feet of these sites and fey creatures are more populous on Eberron during this time. When Thelanis is remote fey creatures grow less prevalent. Some argue that the fey hibernate over this period, but these claims are unsubstantiated by any real evidence. The feyspires that have appeared in Khorvaire come from Thelanis.
Lamannia
Lamannia, the Twilight Forest, is a realm of raw unbounded nature. Untouched by civilization the realm is full of virgin forests, majestic mountains, rolling hills, steamy jungles, festering swamps, and all manner of terrain. Inhabited by all forms of plants, animals and magical beasts as well as a number of lycanthropes who have made a new life for themselves on the plane after fleeing Eberron during the time of the Silver Flame’s Purge. Regions of unspoiled nature floruish in times when Lamannia is coterminous with the Material Plane. Animals breed more prolifically and plants grow more verdant. When Lamannia is remote nature diminishes. Cultivated crops grow poorly and animals product small litters.
Shadowfell
The Shadowfell is a dark reflection of the Material Plane, a plane of death and darkness. It contains Mabar and Dolurrh.
Mabar
Mabar, the Endless Night, is a blackened realm where the faintest spark of light is quickly devoured by the unending darkness. It is a plane suffused with negative energy and the antithesis of Irian. But, just like Irian, the energies of this plane work in opposition to the natural decay and birth of life, which makes a natural source for necromancers to draw power from. Negative energy spells are maximized and all the inhabitants of this plane are immune to effects from negative energy. Positive energy spells are impeded here. The nights grow blacker and more dangerous when Mabar becomes coterminous with the Material Plane. Travel to Mabar becomes as easy as stepping into an area where there is no light, and inhabitants of the plane slip out from the shadows and into the world. The plane becomes coterminous on three dark nights in the dead of night on the nights of the new moon closest to the winter solstice. Consequently when Mabar is remote the darkness seems to sap from Eberron. These periods occur every five years on the nights of the full moon around the summer solstice.
Dolurrh
Dolurrh, the Realm of the Dead, is a hopeless gray waste where mortal souls go after death. It is not a reward, nor a punishment, it simply is. Dolurrh has heavy gravity, it is timeless and people who visit Dolurrh are slowly overcome with apathy and eventually fade, turning into a shade. When Dolurrh is coterminous to the Material Plane slipping occurs between the planes. Ghosts are more common and spells to resurrect the dead often result in bringing back more spirits than originally intended. When the plane is remote, resurrection magic does not work at all and the only way to bring a soul back from the dead is to physically travel to Dolurrh and retrieve the soul, though the native inhabitants of the plane don’t take too keenly to having souls being taken. The shadar kai are said to have originated here.
Xoriat
Xoriat is the Far Realm, a place of madness.
