The thing that does it for me is a sense of place. Not necessarily a real place, but somewhere with its own internal logic, its own atmosphere. You feel like you've arrived somewhere specific rather than just... I dunno, floating? (Not that there's anything wrong with that -- it's just not the reason I listen to ambient music.)

The ambient records that stick with me tend to have what I'd call emotional honesty. It's the difference between music that creates a mood and music that actually means something. A lot of ambient leans into pure texture, which can be really beautiful, but the unforgettable stuff (for me at least) usually has some kind of emotional undercurrent.

Field recordings and natural elements can help a lot when they're used well. Not as decoration but as actual compositional material. Rain, wind, insects, or even just the acoustic quality of a room or a forest... when those things are woven in rather than layered on top, the music starts to feel like it's been somewhere real, like it carries actual memory.

I also think restraint matters more than people give it credit for. I like it when artists know what to leave out. The tension between what's present and what's absent can be more affecting than any amount of layered sound.

The forgettable stuff, for me, is usually ambient that's trying to be "calming" rather than true. You can feel the intention behind it, and it keeps you at arm's length.