"Keep It Simple, Stupid"
That's what my mentor drilled into my head as I took his game design classes. At the time, I think I intellectually understood it, but I never really practically understood it.
It's when I started trying to make games that this rang true. In my first official (video) game designer role, I was given some very strict guidelines...I had a very limited timeline (which, in itself was a hard challenge), I had a limited staff, and I had a small window of time to which I could communicate my vision to the team.