How much I didn't know!
When I made my first game I was cocky as shit, thought I knew everything just because I'd built a few levels in hammer-editor as a teen. Hooo boy I knew nothing about actually making games. I quickly learned just how much there is to think about even with a very simple platformer game.
I had a dude move left/right and jump up, and collide with seaguls which just moved back and forth. It was built in Torque engine and the whole game had maybe 20 seconds of actual content but it took us like 2 weeks to make or something.
I'm quite looking forward to this, although I'll probably only have time to devote maybe a week or 10 days of evenings to it.