I'd like to point out that Metroid Prime was the Metroid game that sold the most, even if it was at least partially due to very aggressive bundling on Nintendo's part.
As to the appeal? Hm.
Well, to understand where I'm coming from, I need to tell you that Metroid Prime was my first Metroid, annd I only played the others later - I only played the remake of the first game, and never played Return of Samus at all.
I think the thing that sums up Metroid's appeal, for me, is somewhat related to yours: not in the hostility of the world but in its alienness, in the sense of discovery and learning. Don't get me wrong, the most memorable moment of Metroid Prime was the first time I fought my way out of the lab where you get the Infrared Visor, but the experience that is emblematic of Metroid, for me, is scanning enemies and looking at their profiles and biologies in the database.
Corruption faltered somewhat in this ,as did Fusion, in that I didn't feel like I was learning as I progressed. Almost everything about the environment was known from the get-go, and the sense of alienness was reduced in turn.
