To many mistakes, imagine Sonic going 3D except crap dialogue isnt the only problem...
Mistake #1: They changed the artists, the game is made a completely new way and this can piss you off if you actually found the first one to be well done!
Mistake #2: Still on the changes, they changed almost every voice actor in the game and revamped Shions look (which to be honest wasnt such a bad idea considering her original character was just an annoying little ******, the kind that always made smart *** remarks to you during class!) They also completely changed MOMO, Chaos, and KOS-MOS introducing the idea that normal realians can GROW and go through puberty only shows how *** the game really is!
Mistake #3: They tried to make the game more difficult and in the process introduced more questions into the game than answers, if you remember the first game, you ended only having a few questions answered, but even more questions revealed. This game pretty much rolls down the same path, even continuing along Jr's life.
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All and all this game isnt so bad, and the major changes found in the game can be explained by a company change, so hopefully Xenosaga III wont be changed even more! [if your really into the series then this games worth it, or if you have time to burn, but if you have other games on your wish list-- go with those!]