But what is it exactly? I hear this all the time but noone can explain to me what they mean? What is so much more rewarding in the other games? I just don't see it.
Hard to pin down really. I think emerging from a lengthy boss encounter to find myself flat broke and covered in Dragon Rot, having won either a single prayer bead or a Memory is a little dispiriting. It feels less like a hard won victory and more like an ordeal survived.
Without wishing to court any accusations of bias for stanning a PlayStation exclusive, I guess I just miss that purely organic design of Bloodborne, where the levels and enemies taught you the mechanics of the area boss without the direct tutorialising Sekiro opts for.
I think that's perhaps a major root of a lot of people's frustration: mobs and bosses are very separate encounters. What you learn fighting the former won't greatly inform the latter. New mechanics are almost always introduced in high-pressure boss encounters, rather than as your exploring the world: sweeps, thrusts, red eye, poison, illusion and lightning are introduced by high-level boss or sub-boss encounters with punishing, aggressive and varied attacks.
If you could hone your skills on smaller enemies with similar attacks thoughout the game world before linking up all that learning in boss encounters, I think those bosses would feel less overwhelming and affect a more natural difficulty curve.
To compound this, the stealth element also discourages combat outside of bosses, as you can down entire areas of enemies without any real duelling combat and areas are clearly designed with this in mind.
These options go out the window in many boss encounters though, and all the traversal, sneaking and baiting skills you develop in general exploration are completely cut off in big boss encounters.
I think this is where the sense of hitting a wall comes from: mob encounters can be easily avoided and even when in direct combat, most smaller enemies can be guard-broken in a single parry or a short flurry of hits.
To add to this, boss mechanics are often singular and sometimes completely at odds with surrounding mobs. Consider Robert's father: an undamageable enemy with an enormous great sword, who can only be overcome by posture breaking him and pushing him over a ledge. He's the first of his kind and almost every enemy around him is mechanically different (close-combat, unarmoured monks or projectile-firing 'rat' enemies - all can be stealthed).
This is where the game design feels most decidedly muddled. Great and celebrated games (including many of FromSoft's) build on this natural philosophy of 'show don't tell' - Sekiro seems to consciously avoid that. It wasn't necessary to have an NPC outside the Church of the Good Chalice to warn you about the Blood Starved Beast's poison attacks and hand you a fistful of antidotes - it was already tutorialised through more manageabe enemies in the run up to that encounter.
I don't know if this really answers your question and apologies for the lengthy post, but to summarise, bosses don't feel like a test you aced because you paid attention and completed the reading, they feel like a calculus paper someone pulled out in the middle a geometry class that your made to fail repeatedly while watching you're GPA tumble in the background.