Their are many reasons why I think Castlevania HD is by far growth for the series.
First off the HD graphics. You can scale the map out to three different camera lengths AND the right analog controls the camera with a flick. Like if you want to look a room over or up a room. But the graphics maybe recycled from old games but they have definitely been given an up res.
Total choice of characters. All the characters play differently and have their own abilities to earn and level up. Jonathan is a tank and the only one with the classic whip. Alucard and Soma use different weapons you pick up, anything from a short sort to a lance or even handguns (and an RPG if you beat the game on normal and buy it) Both Alucard and Soma gain new abilities or spells. Shanoa is half mage half fighter with a special ability that allows for vast shortcuts in the stage and much more loot. Charolette is straight up mage and has to absorb the enemy spells by blocking them with her bind magic shield.
Deep loot and item system with randomized drops and a heavy reliance on luck. All your characters share communal equipment so whenever you play, online or off, you are always adding to the main cause.
Multiplayer, obvious the big one and tied in with difficulty level. To beat any one boss solo is an accomplishment at first. After multiple play throughs of the stage you can usually solo. Bosses are obviously easier with more than one person and I'm sure push overs with a full party.
Hard Mode, after beating the 6 maps (massive maps btw) you unlock hard mode, crazy new items and such. This boosts playtime on an already expansive game. Playing with all the characters, getting the 200 gamerscore I predict about 35-40 hours.
Upcoming DLC with maps and new characters... enough said. This game rocks and I haven't even tried survival mode where you are all hunting each other down. Sorry for the rant I just am suprised at the overall quality and replayability in this game for an arcade title and the sorry reviews and lackluster sales compared to limbo and hydrothunder.