My first jam too.
I think I learned more from other people's games than mine.
The importance of color palette. This is huge. Can make or break a game. I knew this was important but the jam showed me how much.
Also, difficulty. A game too easy is boring, but it's better to lean towards the easier side. It's hard to have good pacing in a game and the worst thing is an unfair stage. I have a level which I thought is too easy so I added more hazards, which made a lot of people to feel frustrated with this specific level. But, had it been too easy, people won't feel something special. And it's better to have a neutral level than a frustrating one.