People who think the Kingdom Hearts story sucks are, without exception, stupid. Being confused because they haven't played the games is another matter.
As for FFXIII, while it's not on KH's level, it's quite sophisticated and has some good character development and presentation. The main problem with it is that it requires a lot of lore knowledge that is only found via reading journal entries and external information... Which is less than ideal, because it prevents many people from ever understanding half of what is going on and what it all means.
While FFXIII's story is excellent overall, except in exlaining itself, it doesn't have KH's depth, lore excellence, well-written characters, etc. KH is just an entirely higher standard, and it benefits from being a very large narrative across many games. While KH is a huge story that was planned a decade ago and is still being presented to us bit by bit, FFXIII is a one-off that they got two totally unplanned (and thus tonally inconsistent) sequels.
