Combos. I don't like them when they're intentional by the developer, they need to be something that you feel like you've discovered on your own. I hear Baba is You is pretty good about that.

I recently observed a game of MtG where a newish player was playing a scry deck, some premade or something, had a guy who would scry every time a creature dropped, and a guy who would place counters every time he scried. He'd edited the deck slightly, added a creature that spawned tokens every time it received counters. Managed to get them all out at once before realising what he'd done; straight up had to ask if tokens count as creatures dropping because he wasn't sure if infinite combos were real. That's a good feeling, because it's something he did, not something that was given to him.

Contrast League or Overwatch or whatever where the devs have specific ideas about how characters should work and will aggressively destroy things outside of that. Or just modern Magic.