"Don't spend much time learning opening variations; the game is decided in the middlegame or endgame".
Many chess books, particularly those who A) were written before the "computer revolution", B) deal exclusively with the middlegame or endgame, or C) present an "easy" repertoire, comment negatively on opening study as a tool for improvement, particularly for amateurs.
While it's possible to become preoccupied with openings to the exclusion of important chess skills, there is no denying that opening study
done right involves both typical middlegames and endgames, and is an essential part of mastery in this age of information. I think everyone who wants to come close to their full potential should try serious main line chess, at least for a while.
I'm still angry that I handicapped myself by following such "anti-opening-study" advice for a long time. It's not merely useless, but harmful. It's not much fun to rely on "general" middlegame strategy when your opponent already knows the appropriate plans for the specific type of middlegame you reach.