I was just playing Metroid Prime, and I realized something about (one aspect of) what makes a metroidvania good, and why some of the less compelling ones we've seen have felt so uncompelling.

Items that give you new moves to access new areas are at the heart of the metroidvania genre. Let's use the example of Super Metroid's red doors: you need to have missiles to open them.

At the start of the game, you can explore a decent-sized chunk of the world before you get your first missile tank (don't ask me why missiles get stored in tanks). But there are a few red doors that bar your way--you have to get missiles before you can go through them.

If these were the only red doors in the game, that would be a little annoying, wouldn't it? You just have to get missiles to get through this one door, but nowhere else are there red doors.

But that's not the case. Even in parts of the game you can't get to without missiles, there are still red doors to make it feel like getting those missiles was worth the trouble, because even if you'd somehow found a way around that first door, you still had all these other doors you need them for.

Compare this to a hypothetical (because all the examples are very forgettable) bad metroidvania where you get an item, use it once to open a door, and then forget you have it afterward. It feels like a pretty useless item, doesn't it? Little more than a key.

This sort of thing does actually exist in some good metroidvanias too, and I do feel it's a mark against them. Think of Symphony of the Night's Jewel of Open, for example. It does nothing other than allow you to open one or two blue doors--and even then, there's no special action that reminds you "hey, you can get through here now because you figured out the puzzle and got the thing!"; you just open it like any other door, as long as you have the jewel of open.