Twilit ridge ~
watches me roll by
calm gray toad.
030. N5 The Tidal Coast - Red
wasted in dank caves,
031. S2 The Big Bridge - Green
Dreary noon ~
hefting barrels 'cross
As a kid I ne'er thought o' this game as a Metroidvania — though that's mainly 'cause it's split into separate levels with a map screen & level progress ( other than getting treasures & all music coins ) not saved 'tween levels, whereas I consider smooth movement through a whole world to be a defining element o' Metroidvanias. As a few people have hinted, it's mo' like a 2-D Super Mario 64, & that's how I would usually describe it.
That said, I very much disagree with Implo's implication that this isn't an exploration game, & find it odd that he would use a comparison to Super Mario World & Super Mario 64 as examples, as those are clearly exploration-based games. These are certainly not fast-paced action games like, say, the original Super Mario Bros. or classic Sonic games.
Also, as Dragon Fogei said, the Metroid games themselves were oft linear. The vast majority o' Super Metroid is a linear path with a few areas you have to come back to later when you have mo' abilities & a few areas you can optionally come back to for extra power-ups — not unlike Wario Land 3. Honestly, Super Mario 64 is mo' nonlinear, as you have mo' control o'er what you can do & can choose not to do: the only things you have to do in that game are get a'least 1 star in the 1st level & beat the 3 Bowsers. Everything else is optional. Metroid Fusion is e'en mo' linear.
Most so-called Metroidvanias are mo' linear than people remember, 'cause making a truly nonlinear game is hard to do well. For 1, nonlinear games cause lots o' problems with difficulty balancing. I ran into this problem with an SMW hack I made years ago: it gave you control o'er which path you wanted to go in, but had its difficulty balanced with the expectation that you would take the yellow switch path, then the blue, & then the green. A criticism I read on SMW Central was difficulty rising too quickly, 'cause that person went straight to the green-switch path.
Thus, Wario Land 3 is smart & does what games like Super Metroid do: it tricks you into thinking it's nonlinear while being linear 'nough to avoid the problems that nonlinearity sometimes causes. If 'twere truly nonlinear, you'd see raocow stumbling into suddenly hard levels & complaining 'bout the unexpected difficulty spike.
I've always thought Kirby & the Amazing Mirror is the best example o' a Metroidvania, 'cause other than a short mandatory linear part @ the beginning, it truly lets you go all o'er from the beginning — you can go straight to world 9 from the beginning. Note the way K&TAM deals with the difficulty curve problem: it takes advantage o' the fact that Kirby games are all-o'er easy, making the difficulty curve not a noticeable problem. If Super Metroid tried to be this nonlinear, you'd get complaints: imagine going straight from Brinstar to Maridia.
Speaking o' which: after the decade or so it takes for raocow's game schedule to lighten, I recommend he play Kirby & the Amazing Mirror, considering he seems to like Metroidvanias & Kirby games.
To be fair, Super Metroid is the same: let's face it, the gun & bomb upgrades were mostly glorified keys. 1 thing I always preferred 'bout Wario Land 3 was that its keys a'least were painted better: while Super Metroid had bland weapon upgrades that just let you open a different colored door, Wario Land 3 has a giant stone foot that can break mountains o' rubble in your way, a flute that calls a snake to come out to act as a rising platform, a wand & spellbook to freeze a lake to create an ice level, & much mo' interesting examples I won't mention since we haven't seen them yet. I'd ne'er thought 'bout it before you brought it up, but the comparison to Monkey Island, which also uses items as glorified keys, is apt.
Also, raocow's playing leads people to forget
you can go back to the Temple to see where you should go next. I expected raocow to play this way, & not his current way o' cleaning up all the bonus treasures before continuing; but in hindsight, that's how he played SMW rom hacks, so it makes mo' sense for him now that I think 'bout it.
An interesting programming note 'bout how linear Wario Land 3 truly is that only speedrunners would care 'bout: the game is so rigid in its order o' power-up collection that what power-ups you have is based on just a simple byte that determines the latest power-up you got, & assumes you have any power-up before that. That saves memory, but means that if you happen to sequence-break, you can collect many power-ups by just collecting 1.