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Hello boys and girls!

I have a future project that I'm thinking about making it into a 2D platformer but I'm wondering how a good platformer should be.
How should it feel? what does 2D platformer games have in common? Is there a game that set an example on how a platformer game should be?

thanks for your time!

In my opinion, "2d platformer" is not a good category as a genre, because it says something about how the game is presented and not what it is about. 2d platformer means that in this game you have a 2d view from the side and that the main character can jump.
Within this category there are games about dexterity and precision (e.g. Super Meat Boy), games about puzzles (Braid), games about shooting (Mega Man), games about exploration (Metroid) and more.

I suggest you first ask yourself, what kind of experience you want to offer with your game. Only then you should decide how to present this experience to the player. If you make a strong decision about the core experience of the game in the beginning, then many design questions that pop up later on can be answered from this directly. The game will be more focused.