Platformer is called a genre, but it's so inaccurate that it's essentially a style of presentation, like 2D. It's woefully useless at describing what kind of challenge you're in for because it includes any games that have a gravitational pull to a single direction. I'm always being sold a "platformer" that ends up being carried by some entirely different mechanic but it has an occasional, trivial hole to jump over every here and there. I haven't even played most Vanillaware games yet because they don't seem to fit my niche, but I hate that I have to rely on my hunch about these things so much since reviews don't even clearly mention what kind of details there is to a game. Usually they neglect to mention everything that isn't obvious from screenshots anyway, like when they go on and on about Super Meat Boy being hard as nails as you kinda expect but won't say what kind of physics premise it's using.
Medium gravity, strong jump force and slight slip platforms that stick on contact by the way is how I would describe it. Contrast that with There's No Time To Explain having strong gravity, weak jump force, but boost momentum buildup that results in a lot more physics-based platforming. Both have the thing that's best explained by not being fixed jumping arcs. You can move in air. It's better to assume so for every game and take Castlevania as a special case, but you have to know what to say is the main difference between Metroidvanias and Classicvanias.
And that's not even exactly what I mean by platforming, it's just the control half of it. Although I guess a good game actually includes all possible kinds of interesting challenge for a given set of controls, so you can just rule them as good or bad, but I'm not really satisfied with that conclusion.