These games are popular platformers that were wildly successful upon release. The theme, design, audio, and visuals are important, but what pushes these games over the top is how they feel. Platformers are often very difficult, which requires the controls to be very fluid and precise.

What makes a 2D platformer good?

I'd say to generalize what makes a good platformer is all those pieces working together in tandem to make you feel like you're progressing towards something. The gameplay has to feel challenging without being too tedious and difficulty curves can't rise too quickly.

What makes a good 3d platformer?

tl;dr: Yooka's worlds are too big and have long periods of nothing. Banjo Kazooie's are small and almost every inch has an activity or item. That's what makes a good platformer. Levels that are methodical and have strong item placement and control of how big they make their levels.