Could someone who's played the game explain the powers of the different characters? I'm familiar with Soma and Alucard, but I don't know a lot about the others. I assume Soma's ability to steal monster powers is pretty much the same, what about Alucard? I read somewhere that he can change forms but from what I remember from SOTN (it's been a long time) that was only situationally useful. I remember turning into mist to pass through grates and turning into a wolf to make long jumps... but that's it.

There's a Quick Look of the game up now. Alucard's form changes appear to be pretty limited. Mist looks to be used exclusively for dodging. (I didn't even notice movement during the form change, so maybe you can't move around with it.) Wolf form looks like it'd be used for moving while staying low to the ground. Based on how it was used in the Quick Look, it doesn't even look like it has its own dedicated button. (Or it just automatically moves you forward, as a slide would in some games.) 
 
I've heard that Alucard, Jonathan and Soma are pretty faithful to their general play styles, with Charlotte and Shanoa playing drastically different. I haven't seen them in action, so I can't say for sure. I read about it in a review.

Yeah, Alucard's wolf form seems to be just his slide move. 
 
What could Jonathan's signature be?  I don't remember him having anything too stand out in PoR...

" What could Jonathan's signature be?  I don't remember him having anything too stand out in PoR... "

The wolf form isn't really a special move. It's just the animation for his slide. His signature move is the mist form but you can't move while in mist form. Alucard can do dark magic if you find the scrolls. 
 
As far as I can tell you can't equip other weapons with Johnathan Morris but why would you want to when you have a whip called The Vampire Killer. His signature move is a block. He can also use sub-weapons and the more you use them the more they level up. Using sub weapons will also level up the whip.

Shanoa I found out has the ability to latch onto those "Magnetic" spheres that Jeff mentioned in the QL.  She then can launch herself in any direction like an arrow.  Not sure about the other characters yet, I only played for about an hour.

Charlotte absorbs and learns spells from enemies with the right bumper. So far I think I have about 10 different spells from just the first 3 stages. Not to mention each one can be leveled up 9 times.

" @kevinski:  Remember, the mist form only lasted for one second in the original game, you had to find a relic before you could sustain it. Maybe you have to upgrade mist form or something before you can do it here. "

I had the chance to play around with all the characters yesterday. Here's my summary of what they can do:
Soma - Can absorb souls (abilities) from fallen enemies. He has slots for 4 active souls (B, B+Up, B+Forward, B+Down), one passive soul, and one soul mapped to RB. Soma doesn't need to be the character to have killed a monster in order to collect it's soul, I've even had them fly at me from offscreen from someone else's kill. He can also use a variety of weapons.
Alucard - Uses magic spells learned from scrolls that presumably drop from chests or bosses (I haven't seen any yet). He has slots for 4 spells(B, B+Up, B+Forward, B+Down). Alucard can turn into mist for 1 second by pressing RB. I assume this is good for dodging hits if you have good timing. From what I can tell he is the only character able to equip a shield. 
Jonathan - Jonathan uses the traditional Castlevania whip and subweapons. Holding RB makes him raise his arms in a guard position, which reduces incoming damage by about 50%. The in game help text mentions him learning martial arts, but I have no idea how that works. 
Shanoa - Shanoa can absorb glyphs from enemies by pressing Up at the right time. I believe there's a visual indication above the enemy's head when that happens, but I haven't seen it yet. She can have 4 glyphs readied at a time. She can use her RB power to latch onto magic fixtures on the walls and use them to launch herself around the room. This lets her take shortcuts and move quickly through areas that require multiple timed jumps with other characters.
Charlotte - Absorbs enemy projectiles and learns to use them herself. She can have 4 spells mapped to B and the directional buttons. The absorb power can also function as a shield against enemy projectiles, even if they're not absorbed.