For me, great platformers have fluid and responsive controls, and either implement a forgiving persisting experience (climbing the environment) or quick and not too far-off resets (level screens).

Platforming can be great in pure platformers, action platformers, 2d or 3d. They may shift but do not limit how stories can be told and how worlds or progression can be designed.

I imagine it can be difficult to balance forgiving platforming with challenges between novice and experienced players. Often, we see alternative or stretch-goal paths for collectibles or challenges, which is a good approach to serve both kinds of players - even if maybe not total novices.

Introducing game mechanics step by step can give good introductions and learning controls, preferably in-game without dialogue, popups, or text-only introductions. They can guide into a natural level and mechanics design progression, giving a natural progression across longer gameplay.