What is it that separates good ambient from derivative garbage? When the genre is based around simplicity and repetition, what makes a release really stand out?
This is actually the right answer.
If it creates an atmosphere and can instill any specific feelings within you, it's good since it does what it's meant to. If you listen and just sort of shrug, then it's probably just derivative garbage.
Pretty much this. /mu/ tries to differentiate between pro and amateur ambient and noise but outsife of obviously lazy stuff like the paulstretched shit, there is little difference in quality between amateurs and people popular in the genre. Listen to what sounds good to you.
You can usually tell if some ambient is garbage, good ambient uses its sampling, synths, or whatever the hell it uses to make you feel some type of emotion. Nothing compares to listening to Lustmord's Heresy for the first time.