You won't be clearing rooms with just one weapon like in most games. This is where the genius lies.
The enemies react so much more to you than in most games in which enemies are mostly automated.
Also, soon after the beginning of the game, you will notice that enemies may not take hardly any damage from certain weapons; this is where the genius takes place. Enemies are often paired up so that you cannot just clear a room with one weapon. In most of the old Castlevanias (and other games in general) you would just equip your best weapon and proceed through the game until you got a more powerful weapon. In OE (Order of Ecclesia) this is not so. You must switch weapons to defeat enemies. So, you may be fighting one enemy and hurting him a lot while dodging another enemy for whom you need a different weapon. It creates so much more strategy and requires a lot more skill. You have 3 sets of Two weapons. You can switch between these 3 sets on the fly. Also, each set can have a different subweapon (this is dependant upon which two weapons you have equipped).
These to me are the most important aspects because the gameplay is so drastically different from most other action games. The animations, music, and everything else is on par or better than anything else in the Castlevania series.
If you don't mind a little challenge, pick this game up. This is possibly the best action/platformer game on the DS, and one of the best i've EVER played.