Dark ambient is like a marriage between ambient and industrial. Is naturally dark and much more interesting than regular ambient. Doesn't feature any cheesy synths, unless made by an ex black metal musician (that oftenly makes people bitch about whether the music is dark ambient or not; it usually is not). Usually doesn't feature any guitars. On those rare cases when it does (like in drone doom), it's definitely non-conventional use.
Much influenced by "musique concrete", dark ambient features recorded sounds, oftenly of non-musical nature.
Dark ambient can have various esthetics. Moaning and chanting, chain rattling (no, that's no ghost) raison d'etre-like stuff, radioactive droning of Fallout soundtrack, heavy pounding sounds of mechanisms, crazy shamanic drum-machine sounds, abrasive noise fragments - all these can make up different moods in dark ambient.
People who don't get dark ambient music, are found scrolling through the tracks in their desperate attempt of finding "where does at least SOME music start?"
There's also a tendency among some cultist freaks to puke up some really lousy synth pads and moans, put runes or pentagrams and some pseudo-religious texts on an album cover and call it "dark ambient", or "pagan/ritual/black dark ambient", or "neo-satanic nazi anti-nazi superficial industrial death dark ambient". Trust me, you don't wanna listen to that crap.
Don't get scared away by Deutsch Nepal's track names. That guy is a sick bastard, but he still makes some good dark ambient.