A couple days ago, I started the Son of Sparda difficulty mode in Devil May Cry 4. I haven't played it for very long, but it has already surprised me. Relatively early in Mission 2, I encountered a group of Assaults, a fairly vicious enemy that was only introduced in Mission 8 on Devil Hunter (Normal) mode. In Mission 3, I encountered a pack of Basilisks, which was the very last regular enemy to be introduced in Devil Hunter mode. As a result, the early missions have felt very different than they did on my first go through of the game. I think this is a great idea, since it has given me fresh and new challenges that I haven't seen before in the game. It is a much better method of differentiating difficulty levels than just tweaking the enemies' AI or stats.
The original Devil May Cry did something similar. If you go through the game on Easy Automatic mode (like I first did), several of the strongest types of regular enemy, including Frosts and Fetishes, do not appear at all. Since I didn't encounter them on my first go-through of Devil May Cry, I was pretty surprised to encounter brand new enemies on Normal mode. I think it was a pretty interesting idea. It gave someone who already beat the game once something to look forward to on a second go-through. In a series like Devil May Cry, where the player is expected to defeat lower difficulty modes before moving on to the higher ones, keeping the experience new is important. Otherwise, the game can get frustrating and dull when moving on to higher difficulty modes.
Goldeneye for the N64 did something kinda similar: it added new mission objectives on higher difficulty settings. For example, on the lowest difficulty setting, the player can clear a certain mission just by fighting his way through to an exit point. On a higher difficulty setting, the player is required to steal some files and destroy some alarms before making his way to the exit point. It made missions play out in a completely different manner, and put the player through more demanding situations.
I vastly prefer this kind of approach to creating different difficulty settings. Not only does it make playing the same game feel different depending on what difficulty the game is set to, it works as a very obvious indicator that the player has moved on to a genuinely more difficult challenge. If anything, I would have liked to have seen the game developers for Devil May Cry 4 hold some enemies or bosses in reserve until Son of Sparda mode.
As a side note, I like the naming scheme for the Devil May Cry 4 difficulty modes. They are just fun.