I understand the Chaos Gods neither as omnipotent nor omnipresent entities, they are beings of the very raw powers of the wrap itself, given form by mortal psychic imprints, which caused the raged currents of the immaterium aeons ago in the first place.
As since the warp is neither governed by time nor space, these entites do not function in the same way the mortals do, yet act nonetheless according to mortals' understandings of emotions, mentalities, and personalities. Meaning Tzeentch is not a master of time -- the warp has no time, nor is Tzeentch is seer of the future -- there is no "future" in the warp. Tzeentch is the Lord of Change, the great schemer, it exists merely to change. That's why he's a patron for both low-class rebels and ambitious noblemen alike -- he has not end goal, he wants only to alter the status quo, no matter the outcome. Which is right and proper for a Chaos god, of course.
And so all Chaos Gods exist with the singular goal to permiate their "essence". To Khorne is wrath, and inspire rage in all beings -- be in righteous anger against oppression or wanton bloodlust, it matters not to him; Slaanesh seeks ever to exceed the limits of sensual pleasures, hence why sloth is basically one of his six circles.
The one that escapes my interpretation, however, is Nurgle -- what sort of mental state is Decay, anyway? What states of mind give way to plagues and pestilences? The old realm of darkness books scarcely explains it better, that Nurgle represents some nebulous concept for the fear of death. That is why Nurgle is so jolly and jubilant, why the worms and flies fester and why he spread so many diseases -- it's the spread of new life it represents.
Another interpretation comes from his rivalry with Tzeentch -- while Tzeentch represents change, Nurgle represents stagnation. Which, wouldn't really make sense for a Chaos God, however, though of course Chaos is inherently self-contridictory and without principals, and just as Gods of Law, so can a God of refuse to change be a God of Chaos.
And that's what made Nurgle so dear to me as a concept -- that he alone is the most subtle god -- not the "I shoot blue flames with me mind" Tzeentch, nor the "I cut flesh with my rage" Khorne, nor the "I r*p* my b*t*ces with my d***" Slaanesh. It is Nurgle who delivers life through death, and prevents death with unlife. There's much that can be done with this concept, much boons to be granted to player-characters, many angles for Nurgle's blessings.
And the overt, gross-out body horrors helps alot with his imageries -- certainly not pleasant, but the clash of the joyous daemons with broken flesh and porous skins definitely make Nurgle the most visually distinct God of Chaos.
