I've been using Unity to make game's for quite a while. Unity is a great tool, but I think that every game developer should know how to make a game from scratch. That's why I decided to start learning OpenGL. this video inspired me to make a Minecraft clone because it seemed like a fun way to get started with OpenGL.
I did this project together with a friend. He did all the procedural generation code and I did all the rendering and gameplay code. We worked on this project for 2 months.
The majority of my time was spent optimizing the game. It is not hard to draw a textured cube in OpenGL, but it is a lot harder to draw thousands of textured cubes. I also spend a lot of time reducing the memory usage. I never had to do things like that while using unity so it was a great learning experience.