Don't ignore older Editions, no system is complete.

Story happens during play, not during prep.

When players can try anything, there is no such thing as balance.

Challenge is dictated by the players, not the math.

Don't use 10 different monsters when just 1 will do.

Original ideas are easier to run than published ones.

Text-blocks are for the DM, not the players.

People wrote these books, learn from them.

Don't control players, react to them.

General ideas are better than specifics.