Right, but the warp zones/whistles and such are basically just the 2D equivalent to 64/Sunshine/Galaxy just openly giving you the option of going from world to world once you've unlocked them.
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Thank you, Eddie Guerrero. http://tinyurl.com/73jgc2r
And I agree that I don't really consider warp whistles to be "encouraging exploration." It just lets you "sequence break." It's not like you can go back after you use them in Mario 3 (unless you're playing the GBA version that lets you replay worlds and stuff). I don't feel like DKC is non-linear either, for the most part. You have a few levels where you have multiple paths, but those are the exception. 2-D Sonic is the only one of those I'd consider to be games that "encourage exploration."
well, there are really two types of "exploration" in 2d games: alternate paths and finding hidden stuff. you have to explore to do both, but they are fundamentally different.