dds2 had 2 big problems imo. first, there are basically 3 kinds of mantras: physical, elemental and ailments. in the first game, they were all equally useful. twas awesome.
on dds2, however, physical mantras were useless after the 1st/2nd bosses and only really necessary to defeat a late boss. ailments were useless against bosses, and rarely had to be used against mobs. to sum it up, you could plow your way through the game using only elemental mantras, with the exception of the early parts and that boss near the end.
the other problem was how inconsistent the game was as far as the pace goes. it started out okay, then slowly picked up. between 20-30 hours the game was awesome, better than dds1 storywise. then, after a certain something happens to a certain someone, the game just goes to shit. you get in a lame dungeon, and afterwards you're in the last one.
you spend +10 hours on the last dungeon without any story events happening. there is no sense of accomplishment as you move further into it. even the bosses are reused.
the main characters on dds2 are worse than dds1's main characters too, and i didn't like how the story unfolded on the second game after that specific event. i can't really get more specific without spoling anything tho.