I'm about halfway through Act II. I have absolutely no idea how this isn't Castlevania. The bosses and puzzles are awesome, the platforming is pretty good, and there are a lot of little clever nods to the history of the series. I like how it takes things about the series and gives them its own spin. But it very much feels like Castlevania. I see what Cox meant about it having Metroidvania aspects, but it isn't a Metroidvania. It doesn't really feel like Metroidvania but takes a few things from it, and has plenty of staples from even the earlier titles.
What I'm actually surprised by are the environments. A lot of the early game has the brown tones, the cavernous places, the dungeonesque places, but the further you play into it, the art direction gets considerably more vibrant. Where I am now is very visually pleasant. The graphics are just great overall. I wish the framerate was a bit better, but at least its playable and consistent.
I really think MercurySteam has an awesome development team that respects the series while making their games not feel like a rehash. But they fit into the overall series perfectly fine just as the Metroidvania games nestled into the series. Going by my opinion of the first game and now this, I'm more than a little excited for LOS2 than I was before, if that's even possible.
One final comment: some of the music could have been a little better, but it still fits with the style of the game, and the music is getting noticeably better the further I get. I've been hearing a really beautiful motif a lot. The soundtrack suffers the same exact problem as LOS1 though -- while the music itself is good, the implementation could have been a lot better. I don't like how it just kind of comes in, plays for a bit and then drops out into silence, only for the same track to return a bit later.
edit: oh and IGN is full of their own shit