I'm surprised nobody mentions DOOM.
I've been playing Doom for almost 2 decades now, and I've been map-editing for three years. Best things I learned that can be applied to general game creation?
1. Traps and baiting players into traps, and making lots of variations on traps (boy, you can make an entirely new game with this concept alone).
2. Enemy placement and type is about as important as it's quantity in determining game difficulty.
3. Meat shields are just meat shields if you don't pair them with anything else.
4. What you can't see (behind the false wall) will hurt.
5. Just because it's green, radioactive slime doesn't mean it'll hurt.
But you can make the player think twice about getting that medkit.
6. Atmosphere's nothing if your player is shooting all the time
7. It's not a clever secret if all the player did is wall-hump every wall he sees.
8. And finally, it's fun to defeat a boss without directions.