In further developing a role for the Far Realm, I feel there are two unique features of that dimension that need to be considered.
First, much of it appears to be biological in nature (whether it actually is or not is up for debate, although I prefer to think that the forest of giant hairs should actually be a forest of giant hairs).
Second, it has a clear connection to psionics, as much of its inhabitants use psionic powers. Furthermore, in 4E, one of the several possible explanations for psionic power is that it originates in the Far Realm.
To address the first point, take the phenomena of mutation into account. Mutations related to exposure to the forces of chaos are manifestations of creative forces that seek to create new entities. Mutations related to exposure to the Far Realm, however, seek to impose another set of physical laws upon the world, transforming one kind of natural creature into a being from another universe. Creatures subjected to this type of mutation usually die, as their bodies become unsuited for continued existence in the universe.
Perhaps a "star" from the Far Realm falls to the mortal world one night, bathing a town in its glow, before zooming back up into the sky come morning. The light carried with it a bizarre radiation that eventually caused horrific mutations upon the living and inflicted severe, fatal birth defects upon the unborn. However, this light did not wreak random chaos upon the morphic structure; instead, it attempted to selectively alter those who were exposed to it in an effort to transform them into the creatures of a reality that never fully came into existence. The only reason these mutations were fatal was because they are not suited for the physical laws of this reality.
These mutants may have fared slightly better in the Far Realm, but they still would not be safe. The physical laws of that dimension are localized and inconstant. At any given moment a god-like invader from another sector of the Far Realm could appear and rewrite the laws of unreality, making the local inhabitants suddenly unable to exist. Those creatures who somehow avoid death are transformed (take the kaorti for example).
To address the second point I mentioned earlier, take the difference between arcane magic and psionic "magic". Arcane magic functions by manipulating the physical laws of the universe. Psionics, on the other hand, might possibly violate reality. If the wizard is learning how reality works and takes steps to manipulate, the psion may view the world as nothing more than a dream that can be altered as if by a lucid dreamer.
The Far Realm, which is a place beyond known reality, does not have consistent physical laws that the wizard can manipulate; neither does it have gods or divine power. Instead, it has alien beings that manipulate their dimension in the same way that a lucid dreamer manipulates their dreams. The nature of reality is not determined by physical laws or the decree of deities, but by those beings whose will is powerful enough to override the will of others. The Far Realm might thus be a place where there is no distinction between the physical and the phrenic.
So far I've assumed that the Far Realm appears to be composed of biological matter, but what if the nature of the Far Realm actually depends upon the observer? Perhaps the god-like entities of the Far Realm seek to shatter the will of those who dwell in it so that no one can contest their dominion (other entities may dominate creatures, perhaps by causing them to perceive the world in ways that insure they act as required). They might do this by willing the substance of the Far Realm to take the form most terrifying to a viewer. A person terrified of parasites might see a massive worm penetrate their skin and crawl inside him, somehow leaving him visibly unaltered even as he feels the thing completely filling-up his insides. Another person terrified of spiders might see a giant eye open in the "sky" before growing spider legs and clambering towards him. These are not mere illusions; even if each person sees something different, each perception of the Far Realm is equally real, even if they contradict each other. These altered forms of perception might persist even if the characters escape: the character afraid of parasites, for example, may be able to see the parasites in the natural world around him as if they were the size of dogs.
Gibbering mouthers might be born from creatures who succumb to their greatest fears within the Far Realm. A random farmer might somehow be drawn into this dimension of madness and see nothing but that which incites the greatest terror within him for an infinite distance in all directions. Somehow, perhaps by the will of the entity inflicting this universe of terror upon him, the farmer does not die. Instead, his form and identity begin to dissolve in unreality. His body becomes formless goo, and he loses all memory of his life. The only act of will he is able to express is the formation of new mouths through which he can let loose howls of terror. Finally, and most terribly of all, the creature that was once the farmer forgets what incited this fear in the first place without the terror fading away, for all it now knows is unending, sanity-pulping fear.
Even if a person can somehow survive entering the Far Realm, they should not want to stay. If you really want to impress this horror upon your players, find out in advance what their greatest fears are and, once their PCs enter the Far Realm, describe an entire universe where their most irrational, primal fears are horrific reality. Of course, such an approach could easily backfire and make your players hate you, so you might not want to attempt this (heck, I myself can't help but think momentarily about what kinds of microscopic bacteria are swimming around in the tea I'd drinking right now). If someone DID do this without an incident, though, I imagine it would be fairly satisfying to have your players say "We are NEVER going back there again. EVER."
