So I've been holding back on commenting on Mirror of Fate until some real concrete gameplay footage shows up, and now that we've seen the full demo from beginning to end, I'll chime in.
In short, I'm sold.
No, it's not exactly like the Metroidvanias that we all know and love, but bear in mind that Castlevania wasn't originally a Metroidvania at all. The series evolved with Symphony of the Night, and now it appears to be evolving again with Lords of Shadow. While I wasn't a big fan of LoS as a game, I like where they're going with it. It really does seem like the devs have taken the many criticisms of LoS into account - the game actually FEELS like Castlevania now (I cheered when Cox said 90% of the game would take place in the Castle) and 3D combat seems to flow remarkably well for a 2D platformer. The enemies are quite Castlevania-ish and the environments are just wonderful - I loved the big cathedral where Trevor picked up the boomerang. The 2.5 effect seems to work remarkably well for this game.
If the full game maintains that, that's good enough for me. I'll admit, though, a couple of things have taken a chunk out of my anticipation - I do have to admit that the lengthy combat with every goddamn enemy looks like it could get quite tedious during Metroidvania-style gameplay. It seems as though their solution to that is to just remove all of the enemy encounters whenever you're exploring, and trigger them at specific times. Okkaayy... I guess. Doesn't feel quite the same, but I can deal with it.
Something I am definitely going to miss are the RPG elements, which appear to be completely gone from this game. Unlocking combo moves is just not the same. I liked breaking apart walls and finding random slices of pumpkin pie, swords, and other doohickies. It really added some replay value, and made all of your "exploration" feel meaningful in a tangible way. Now it appears that running around the Castle is more or less a complete waste of your time. I dislike this, but not so much that I can't get used to it.
Not huge on the inclusion of light and dark magic. Feels out of place in a CV game. But again, it's not a dealbreaker, just a nitpick.
In short - if they iron out the framerate issues, continue to impress with the environments, get a better soundtrack and emphasize the Metroid-style gameplay I think this will be the best Castlevania game in a long time. Who knows? Might even rank with the DS/GBA trilogies. Time will tell. Meanwhile, I'm excited.