Although they wernt all side scrollers, the Legend of Zelda games used a simular "power-ups as keys" approch to much success.

Iron Chef Carnage: there's nothing inherently wrong with keys that act only as keys. If you have some reason to require the player to visit certain areas in sequence, then they can make good ways of enforcing that sequence. So long as you don't make the key-collecting segment too onerous, you generally won't annoy your players too much.

I actually think, though I have no studies done to back this up, that abilities that only are really useful as keys are worse than keys that are explicitly useless except as keys. It's a matter of presentation: for the latter, the player knows that all they're really getting with the key is access to a different area, and they're fine with that. For the former, it's not nearly so clear; it seems like they're getting a new ability, which should be useful somewhere, but it isn't, really, except over here to get past that obstacle. That's deceptive game design. It's like being given a sword labeled "Sword of Awesomeness +10", which is so heavy that it takes you a full ten seconds to swing. You can use it to break down walls, sure, but its name belies its utility, as it's utterly pointless as a weapon. The player will end up trying to use the sword/ability in many situations in an attempt to find a use for it, and will end up concluding that it's simply not worth the effort. That time spent was essentially wasted, and probably not very much fun; therefore, it's something to be avoided.

Leiavoia: two things. First off, with the "limitation of keys" thing - I don't personally like that idea, as the player will have no idea going in that any one weapon will be more useful than another. You'd essentially end up with players picking weapons for the first descent either randomly, or based purely on what they're most comfortable with, and then, once they know what they need, returning to the start to equip what's actually useful. In other words, the tradeoff decisions you envision can only be made by someone who has foreknowledge of the regions they'll be traversing, which basically destroys the exploration focus of the Metroid-style genre.

Secondly, with having non-obvious uses for abilities. Good idea. Some examples off the top of my head:

1) Freezing enemies to use them as platforms. This is moderately obvious to people with past experience with "ice" weapons, but the first time you figure it out is fun. Another ice-ability in the Prime series is using missiles to shatter frozen enemies with a single hit.

2) The "shinespark" ability of the Speed Booster in the Metroid games, which I don't believe is ever explained directly to the player. Certainly it wasn't in Super Metroid except by a creature who mimicked the actions you had to take to use it.

3) Just having items that aren't required to reach new areas, and clearly so. Rewarding the player with items that are useful in their own right reduces the feeling of "find the silver key to open the silver door".

AM I wrong in believing that the Megaman series (the older ones, on SuperNes or GameBoy) did something like that? You could take out ANY ennemy with ANY weapon, including the basic one, but it would be easier if you followed some sort of sequence, like first water, then fire, then ice, then electric, then boomerang, then cut, then... But you could begin the chain wherever you saw it fittest...

In general, I've noticed that the real long-term replayability in many singleplayer games comes not from developer-set difficulty levels, "100% completion" goals, or other things like that, but from player-set challenges that specifically remove some helpful thing from the list of things that the player can do. For example, I've played through Megaman 3 from time to time using only the basic gun, and believe you me it's a heck of a lot harder that way; each boss only takes 1 point of damage from the peashooter, compared to up to 6 points from the weapon they're weak against. Similarly, in Super Metroid you can limit yourself to 3 energy tanks (you need at least 300 health to survive a scripted combat at one point), or in Symphony of the Night you can try playing the game without equipping any weapons. The most classic challenge is playing through Final Fantasy 1 with a party of four white mages. These challenges can significantly change gameplay, creating a new player experience. Of course, some games are more amenable to challenges than others. If you force upgrades on the player, for example, then they can't very well avoid them, can they?

With that in mind, I'd say that games in this genre should be reasonably challenging the first time through, perhaps easy for repeat players, but with many different ways of accomplishing goals, some of course more challenging than others.

Quote:2) The super-jump ability in Symphony of the Night. Bat form is a requirement to get it, so you can already fly; this just streamlines the process.

Yet they still found a way to use it outside of just streamlining the flying process: you can juggle the librarian's chair from below using this skill, and you get items for it :)