so among the complaints I've heard about the GW is that the Negative, Shadow, and Vacuum are too similar. All are dark voids. Now, obviously this is inaccurate but I think it's worth looking at how these three (and any other dark voids) can be differentiated.
To start, I think Negative is Hungry Darkness, Shadow is better as the mixture of Positive and Negative energy, the place of illusion as well as the subconscious, and Vacuum is like wild-space that goes on forever.
What does having 3 darks offer the game?
Another great topic sci. I hope you find a way to differentiate vapor and steam too.
I think I remember Shadow is sometimes used for fast transport (as in shadowdancers.)
Vacuum is absloute nothingness. Theres nothing there including light, and where there's no light, there's no darkness. Here's a great concept for planars to baffle primes
Negative energy is, however, darkness in its raw form in opposition to light. Just like negative energy is a form of energy like the positive, so is darkness a form of illumination like light.
Shadow plane as it is stands out way too much in my opinion. First of all it does not have a parallel. We could try to explain it as the combination of the negative and positive energy planes but this doesn't suit its feel. this is why I like shadowfell and feywild better (they are pretty much the only good ideas in 4th ed.). I like to conceptualize them as the negative and positive reflection of prime worlds in ethereal. Perhaps they exist as an in-between for prime material and ethereal.