This isn't one of those posts that has a thesis or an opinion or clever wit and finery. It's the kind where I just analyze the factors behind Wario Land II's invincibility mechanic and think to myself, in my head, if it's all really worth it.
One of the most idiosyncratic features of Wario Land II is that Wario possesses no concept of health or dying. Instead, his foes' attacks have unique results. A hammer-wielding foe may smash Wario into a bouncy spring, a flame-thrower may set him alight, or a bird-bee's sting may cause his head to balloon, carrying him off the ground. It's all rather... Looney Toons-y. In a very fun way. It gives appeal to danger: how can I hurt Wario in a useful way?
From a game design perspective, these provide a twist on a time-honored tradition: the power-up. Each of Wario's wacky new states brings a slight variation to his rules of control, opening new avenues of progress to the player. The correspondence of enemies to power-ups isn't new - Kirby uses the same dynamic. So it's the dispensary method that has to be mined for gameplay - what new interaction does it facilitate beyond what is accomplished with ?-blocks and enemy drops?
1.) It changes the way the player interacts with the power-up
Since these power-ups are enemy-triggered, it's natural that they can impede progress as easily as they can benefit it. Really power-up is a misleading label, since the changes to Wario's abilities are more a lateral move, enabling and disabling him. This encourages the player to look at why they need this new ability - what are they hoping to gain from it? In Contra, grabbing the Spread Gun is never going to hurt your progression, so it's a why not - it's not a decision at all to take the power-up, but a linear progression. The focus isn't on what you're going to do with it, rather it's on how it's different from what you already had. It's a character-centric notion of powering up. Wario Land II offers situations where a power-up may be beneficial or detrimental, so you're less concerned about nabbing the new ability, and more concerned about discerning how you might use it. If you get smashed by a hammer dude, is there somewhere you're actually going to spring up to? Or should you be moving down? This makes for a progress- or environmental-centric notion of powering up.
2.) It changes the way the player interacts with the enemy.
Tying an enhancement to an enemy behavior means that the player has to trigger that behavior to progress. They don't need to attack the enemy - rather they need the enemy to attack them. The enemy is not static like a ?-block, nor is it fully unpredictable. It conforms to movement rules defined by patterns and intelligence - movement rules that can be manipulated by the player. This is a form of indirect control, as the player is using their input to try to elicit a particular output. In Wario Land II, this tends to take the form of drawing enemies from their spawning area into a region where their power-up-attack is needed. This is more complex than simply hitting a switch or pushing a block, as it engages the player in the activity of transference.
Wario Land II is a sophisticated game that succinctly presents much of what makes platforming enemies fun. In some sense, it is for enemy interactivity what Super Mario Bros. was for environmental interactivity fifteen years earlier. It stands as a clear testament that a platforming hero needs to do a whole lot more than go through a world to keep players engaged.