The series relies heavily on backtracking and near random happenstance to find where to go. The game often gives you a key and tells you "somewhere in this world is the key for this door."
See to me that's like a good sales pitch. You're pretty much listing off everything I like about Metroid. The appeal is very similar to that of a puzzle or a mystery with some discovery as well. When I bomb and find a passage I feel like an archaeologist finding a secret passage.
If if you don't like Metroid Prime it is at least is a very innovative game, so much so that it gets flack from rubes for not playing like an FPS because it looks like one in screenshots. What does Wind Waker do that is is even remotely creative or interesting? It's basically Ocarina of Time with cartoon graphics, a big blue ocean of nothing for an overworld, and an irritating time consuming method of getting around. What makes it exceptional?
One thing I find very frustrating about being a Metroid fan is that more than probably any other videogame franchise it is constantly criticized for what it's not. And Nintendo actually CATERS to the people who hate the series by making games for these people. Metroid Prime wasn't an FPS and it got tons of flack for that so they made Metroid Prime Hunters. Meanwhile the series gets criticized for being too obtuse and having too much backtracking so they make the games MORE linear and put obvious hints in and eventually get to the point where we get Other M which is more or less a linear action game. It seems to be designed specifically for a target audience that dislikes Metroid.
What other series has to deal with this where the concept is compromised more and more to appeal to people who don't even like the games in the first place? Does Nintendo **** around with Zelda and Mario the whole time to appeal to non-fans? But then Mario and Zelda have more universal appeal and maybe that's the problem. Metroid doesn't but Nintendo can't really cope with one of their franchises being a profitable cult game. Not only do I feel that I have to defend my Metroid fandom against haters but that I have to fight against the creators of the games themselves. It's like the fans that made the series successful in the first place are punished.