If we're talking technical stuff, I've pretty much learned the basics of unity and C# without any prior knowledge. I've also dabbled with music production in Bosca Ceoil Blue, as I really wanted every asset of my game to be handmade by me, which meant I have to create some background music. It was super fun to learn these tools!
Apart from the technical stuff however - I've realised you gotta make your games as accessible as possible. Half of my time spent on the programming and design was thinking how will players react to a feature I'm working on and if they even enjoy it, or is it something that's cool only in my eyes but makes no sense to someone just playing it for a couple of minutes.