The flashback mechanic in Blades in the Dark. I love playing out intricate plans but I’ve been in games where we’d spend one whole session planning for every two sessions playing and while it can be fun it can also feel like a huge waste of time. (And there I wasn’t even the GM, for whom in my experience long planning sessions are much worse.)
As a GM, I love the monster design and encounter building guidelines in D&D4E. I ran a combat heavy campaign that wen’t to level 14 or so, and with the exception of one or two encounters every single combat I ran was fun, tense, and exciting for the players and fun for me to run as the GM. I never had to worry that I had picked enemies that were too dangerous or that a creature that was supposed to be a tough opponent would be a pushover, because the challenge ratings just work. If I want to give the players a tough fight they will probably win, I can give them that by following the guidelines and then try as hard as I can to beat them with what I’ve got. I never need to hold back or fudge because the balance is already there. It gives me exactly what I want for a combat focused game where tactics and player skill are part of the fun.
I also love the playbooks in Beyond the Wall. I’m not sold on old-school D&D as such, but man those playbooks. They illustrate character archetypes, they give you hooks, they create character connections, and they set up the setting. Yes, this is standard for PbtA games, but with the random tables everything becomes so quick and easy.