That is indeed interesting to see how ff13 got that high score in japan. I wonder what the standards are and whether many of them are otakus. Or perhaps they do value the story and the presentation most, which is surely impressive in its own merit.
i think it's the story thing (visual novels are one of the best selling genres in japan, equivalent to FPS games here), but also even in mechanics ff13 did some interesting things with its paradigm system. for instance, healers could not attack at all: their role was to heal and that was it, unlike healers in most rpgs. similarly, a "tank"'s role was to absorb damage, and that's it, tanks could not attack. this made parties much more reliant on each other and on the composition of "classes" in the party than in most rpgs
it also added class-changing *in battle*, which ffx-2 had but not to the same extent: you have to change classes about 100x as often in ff13, in ffx-2 you could change classes in battle but you never really needed to because every class was more well-rounded. whereas in ff13 it was more like this: boss about to unleash a super attack? change class to tank for everyone to absorb it. are you heavily damaged or someone dead? change someone's class to medic to heal, then change back when the healing is no longer needed
so it required more strategy than just the 'press A repeatedly to win' battles of most of the recent final fantasy games (basically ff7 through ff12). so i can definitely see the appeal in japan, even if i personally didn't like the total lack of exploration, towns, customization, weapon variety, mini-games, puzzles, and so on ("streamlining" they called it)
regarding sales rates, here's sales total, not sales by week:
http://vgsales.wikia.com/wiki/Final_Fantasyfinal fantasy 1 = 0.4 million
final fantasy 2 = unknown
final fantasy 3 = 1.4 million
final fantasy 4 = 1.44 million
final fantasy 5 - 2.45 million
final fantasy 6 - 2.55 million
final fantasy 7 - 9.8 million
final fantasy 8 - 8.15 million
final fantasy 9 - 5.3 million
final fantasy 10 - 6.6 million
final fantasy 10-2 - 3 million
final fantasy 12 - 5.2 million
final fantasy 13 - 6.2 million
so it sold better than ff12, and almost as good as ff10, and far better than ff9. it didn't equal the heights of ff7 or ff8, but sold far better than final fantasy 1-6, which we usually consider the classics. so basically it's the
4th best selling final fantasy game out of all 13
in any case, maybe ff13 isn't the *best* example of what i meant, but it's still *an* example of a game that some people believe is a good game and love it and some people swear it's one of the worst games they ever played