I was expecting it to more or less follow the plot of Dracula's Curse, with Trevor setting out on his epic journey to the castle, meeting and teaming up with his new allies along the way to eventually confront Dracula and put a (temporary) end to his reign of terror.
I was not expecting nearly the entire first season to center almost exclusively around how much Warren Ellis hates Christianity, which, even if that's your thing, has nothing whatsoever to do with the plot of any Castlevania game, ever. The second season was nearly as bad, with the majority of it getting wrapped up in vampire politics, Carmilla manipulating everyone, and ultimately making the main character of Curse of Darkness her personal bitch. Meanwhile, Dracula just sat and stared at his fireplace in a state of complete apathy, and Trevor, Sypha and Alucard literally spent half the season sitting in a library, with the show barely remembering to come back and check on them for a couple minutes per episode, before going back to the Transylvanian equivalent of some stupid reality show.
Having said that, the (one) episode where the heroes finally stormed the castle and fought Dracula was flat out incredible. But that only served to underscore what the show should have been doing, all along.
My bottom line: the animation was decent, and the voice cast was terrific. But as it is, I think most of the series has been a huge missed opportunity, spending far too much time on boring side characters who weren't even in the games, and not nearly enough time on the ones who were. If they'd followed the games more closely (which Ellis is on record as not having bothered to play or research any of them), the series could have been a lot better.