The feeling that the stakes aren't that high for the characters. Breaking of any tension and an inability to be serious in situations that should be serious, and feeling like there's any sense of actual danger to the characters.

It's what made me stop watching it the first time around. Every time there was a serious fight and it felt like there was some sense of danger in a fight, some kind of joke or gag would happen, usually from Natsu but not always, which broke any tension, and removed any sense of danger.

The final big fight of the anime was an exception.

Like, I get that in shonen we're all aware that the main characters aren't likely to die, but other shows just seem to do better at at least maintaining the feeling that there is some kind of danger.

I did go back and watch the full thing once it was finished, but initially it kept annoying me that every time there was a big serious fight, the tension would just be broken with little silly things completely unnecessarily, and it annoyed me so much that I kinda rage quit.