I played the game for about 6-7 hours yesterday. I wasn't immediately smitten with it, but once I got into it, I couldn't put it down.
I was thinking about it this morning as I was taking a shower and I've personally come to some conclusions about why this game is so awesome:
1) The enemies actually have resistance/weakness attributes and it actually matters. In the CVs that I've played (AoS, PoR), you could hack your way through the game with one of the more powerful weapons against nearly every single enemy. For example: Claimh Solais. Once you obtained this weapon, you basically didn't need to use anything else and all strategy -- for most of the game -- was thrown out the window. The same is also true for the soul system as well: it didn't really matter which soul you used once you found a powerful one.
In OoE, you are constantly switching glyph sets, even when you revisit parts of the game which should be a cakewalk. Of course, the beautiful part of this is that they put in a very simple - yet powerful - system to set up sets of glyphs. The sleeve system empowers the player and makes the experience fun, strategically. If they had forced players to go into the start screen (i.e. no sleeve system), this game would not be as enjoyable. As an example, I always thought that one of the shortcomings of MGS3 was that it could have played up the role of camo a bit more and made it more strategically fun if it wasn't such a pain in the ass to keep switching camo by going into the menu.
There's also some variety in how dual wielding stacks. As I said: this game empowers the player through the superb control system and introduces an element of strategic action, but in a way that doesn't punish the player by having to take the experience out of the action and into the menu system. It makes it a joy to play.
2) The glyph union system, while limited, is pretty awesome. I would have liked to see more distinct combination types. For example, if you combine a non-weapon type glyph with a weapon type glyph, you get the same animation and same attack characteristics, regardless of which weapon type you combine it with. It would have been cool if you ended up with a greater variety of combinations to experiment with. For example, combining the ice glyph with a sword type glyph would yield a different result than combining the ice glyph with an ax type glyph.
But still, while it's not very deep, it's deep enough that it adds variety to how you configure your glyphs.
3) While the game is not "OMG I'M GOING TO THROW THIS ACROSS THE ROOM IF I DIE ONE MORE TIME" hard, it's definitely not as easy as the other CVs I've played. The key is that it's not hard in a cheesy way. At all times, you feel that Shanoa is sufficiently powerful; I would summarize it as "it's hard in terms of strategic action". It's not mindless hack-n-slash.
Also, the item system is very limited (in a good way). There aren't absurdly powerful items in the game (yet) that nullify the need for player skill. This is a good thing. Healing items are VERY weak, in general.
4) As an extension of (3), I think they did a good job balancing the game. Shanoa's life meter isn't absurdly high to the point where she can just take damage and slash her way through bosses. Also, using MP for attacks adds to that sense of balance: you can't just slash your way through with your most powerful glyph. You need to consider strategy. The glyph union/heart system is also a nice touch since it means you can't just spam your most powerful attacks and expect to win. At least to this point, at every boss encounter, I've run out of hearts long before the battle has been over.
Conclusion: this game is solid, fun, and ultimately very satisfying. It's everything that's right about gaming. If you have a DS, you owe it to yourself to get this game.