People taking it too seriously are one thing. Another is people not taking games seriously at all. As in they just come to chat and barely engage with the game and won’t learn the rules even after multiple times playing. They will also be super lax with the rules and basically do whatever.
The worst offenders are the ones who are super lax with the rules, taking back turns etc, unless it is against themselves. Maybe I just had some bad luck. My current board games group is super fun though.
As for games I dislike games where the good choices are too obvious. It doesn’t have to be heavy to achieve that imo. But it has to have a minimum of complexity for me to enjoy it.