*SPOILERS* Incredible game with almost no flaws.
Now for what makes the game amazing. The controls are spot on, and just flow after 3 minutes, tops. The story is incredible, and though I never played Aria of Sorrow, that doesn't matter. It may be a direct sequel, but it's its own game, that anyone can jump in and play without playing Aria and still get the story.
The endings are a bit confusing, although each is well done. Getting past the first "bad" ending is fairly easy, although you won't be able to do it unless you have a specific soul, which is annoying. Now, the second "bad" ending is a real pain to see coming. Unless you've either played through the game, used a walkthrough, or got REALLY lucky, you won't avoid this ending. Even after you do the required steps to avoid it, it still looks like you get the "bad" ending, which is annoying at first, until you realize how effectively it set up the true ending.
The weapons system is very nice. There are about 10 weapons that you can choose from, each with advantages and disadvantages, and they can be upgraded for free by infusing a soul into it! Somewhere, Hammer cries. You find upgraded weapons throughout the sanctuary (gotta love video games), and the only problem I have with upgrades is that some of them require souls you don't see until the very end, when you're in Hell. Add to it that they are rare souls to get, and hard to kill without taking hits or getting irritated, and you're most likely stuck with that weapon for the rest of the game.
My fusses are mainly minor, and really do not affect the game in any way. The only major complaint I have is that this game requires a walkthrough. After killing the boss in the top right tower, the game becomes confusing, vague, and downright annoying. If you are satisfied killing Dario at the end, this should be about a 10-20 hour game. If you are satisfied becoming the Dark Lord (which you shouldn't be) and killing Celia, this is about a 15-25 hour game. If you want to defeat the Creature that comes of Dmitrii's failed Soul of Dominance, it's 100 hours without a walkthrough. With a walkthrough, this game is much easier, and will take tops 15 or 20 hours to complete with 100% if that is your goal.
Also, just as clarification, I say the difficulty is Hard because I know that unless you're one of these "purists" that insist on playing a game through without help from a walkthrough, this game is incredibly difficult. The bosses themselves are nothing hard except for 2 or 3, but figuring out where to go and how to get the "good" ending you may as well be Jesus Christ reborn if you can do it in under 30 hours with no walkthrough or help your first playthrough.