Just everything, honestly.

The battle system for starters is constantly asking you to think smart. You've a gauge that you have to keep centered to be "in the flow" such that you can't blindly use the best attack every time. You always start every battle with full health and mana - so you never have to ration between fights. Which adds up that every fight can be difficult. There is no cannon fodder.

You level up as the story progresses so you can't simply grind to a higher level. There is a level system but it's more a skill mastery one It respects your time in that you will never need to grind for anything.

The puzzles are difficult enough to be challenging but not so much that you will ever need a strategy guide.

The story is relatable without being convoluted. Each character has realistic goals, and interests. There is no clear "I'm evil because fuck you I'm evil". There is no clear good guy either, and you will see what I mean if you finish the story.

The world makes sense and has no suspension or disbelief moments. Speaking of it's a world in which both Dragons and Giant frickin robots are both natural, and they make sense.

I absolutely loathe when a JRPG Battle System demands that I either save everything so that I have MaNa for the boss fight, or makes it so that you should just click 'Attack' every time with no reason. I also loathe random battles. This has none - you choose which monsters to fight, and they don't come right back.

I literally could not put it down for any length of time until I had hit 100% of everything. A tall order for a game that took me almost 100 hours when I'm a full time adult with kids.