It's a joy to discover HP, TP and Capacity ups as a result of exploring the world.
When I thought I was about to go somewhere where I expected an invisible wall, it's always nice to see,
somebody actually cared, and gives me some bonus, for trying to figure out how to get there.
Or when randomly attacking a wall, because a completely empty room (besides an enemy) looks pretty weird,
it's a joy to get a bonus.
It makes running around the castle and exploring it actually fun.

Some are hidden surprisingly well, e.g. there are HP and MP upgrades above the drawbridge to the castle, off-screen.

I found that one by just playing, which blew my mind, as there are multiple such secrets scattered all around. For example there's one hidden on the train (in a similar fashion) as well.
That's what I like.