Game Design is a fairly new field. I've been a professional game designer since 1994, but I started making games when I got my first home computer back in 1982. The first time I even encountered "game design" as a discipline was around 1999. Those of us who have been around for a while know that we're all still learning.
I've been a design lead on a huge variety of games, from co-op action platformers to hardcore military simulations to one of the first big free to play MMOs, on a bunch of different platforms. I've made some great games and some that didn't turn out quite as great as I'd hoped. But that's how we learn: it's the pattern of growth, and how we as designers develop best practices. I'm creating a series of short podcasts to spend a few minutes talking about just that, on Play Patterns & Practices.