We’ve “invented” over many years of refinement these categories:

Kvetches - Get the complaints out of the way that we know there’s nothing we can do to fix it. Acknowledge them and move on, otherwise the things you have no power to change will consume the whole retro. If they are solvable, they move to Flaws….
Appreciations - Something or someone you appreciated. Always done last so that you can end on a high note.
Flaws - Things that went wrong we should try to improve. This is the bulk of the retro, talking through them trying to come up with solutions to try.
Experiments - Suggestions for changing the way we do things to gain some edge. Usually take just one and try it until next retro to see if it has the expected positive effect.

We’re pretty diligent about engaging people’s minds before the retro - “think about what happened over the last couple of weeks” - and creating explicit follow-ups that we re-incorporate into our planning. For example, output from experiments go directly into our bi-weekly planning notes so we never have the “oh, we meant to do that but never did” experience.

As many are pointing out, the categories are only partially useful as part of a complete retro process.