Is it because of its plot ? A Belmont who becomes the Lord of Darkness, the presence of Satan as main antagonist, etc.
The plot is not doing it any favors, that is for sure. Gabriel becoming Dracula didn't make much sense at the point of the normal game ending. In fact it was all pointing in the opposite direction. It is as if the end point was demanded, and the story had to inevitably end up there. So in a series of cheap and shitty writing twists, via DLC, we ended up there somehow.
There is also the whole tricking the fanbase by heavily implying that Dracula was in the game, and Gabriel would fight him, in the trailers. Par for the course for Cox though, it would seem.
Is it because of its music? Oscar Araujo has been critisied for creating a "blank" soundtrack, at the opposite of what the series is known of.
Opinion wise, I like the old style of mixed genres and unique sound tracks. Fact wise, Araujo's tracks were completely movie style. Game composition is NOT the same as other types. You're setting the mood for the typically indefinite duration of the segment you are currently in. Araujo's music just changes moods so much within the single tracks themselves, that you will often get mismatched moments. Hurried epic, while slowly shimmying on a cliff side, and quiet calm while racing down a collapsing platform segment.
Some of his tracks aren't that bad, but he needs to understand what game composition is instead of trying to shoe horn movie compositions into it and hope it's good enough.
Is it because it's using stuff for other games? Shadow of the Colossus bosses, God of War gameplay, etc.
I personally don't see a problem with this, as games barrow concepts back and forth all the time. However, when you try to do this, make sure that you do not completely fail and cause something much more inferior to the original. Case in point: the titan battles.
Is it because of its bestiary? No Medusa head, creatures from Lord of the Ring, bizarre vampires, etc.
One of the things that makes Castlevania stand out is it's interesting and imaginative cast of baddies. I don't mind so much the changed vampire image, but I do find it a bit insulting and disappointing that the cast of normal enemies has been downgraded to blatant Lord of the Rings ripoffs. There is a place for Lord of the Rings, but that place is NOT in a Castlevania game. There are plenty other generic medieval titles that can use this. I like Castlevania because it is not generic, and the same as all the others.
That is not to say the entire bestiality was awful. The rotting corpse guys with detached heads, and the casket spiders were pretty awesome. We did at least get some magically animated armors too.
Is it because of another thing? Explain, propose and discuss here, please.
I think another thing that set fans off in the wrong direction, was the unwarranted self assurance of the whole thing. Cox and his band of men gave the impression that there was something terribly WRONG with how Castlevania is, and that it is incompatible with the current generation of game players. Most of us would completely disagree.
As a whole, Lords was NOT by any means a BAD game. In fact, gameplay and graphic wise I really enjoyed it. It's not perfect, but a high quality, and perfectly serviceable, game. The contention comes into play when you decide to slap the Castlevania title on it. Once you do that, the bar is raised high, and many cannot seem to meet the mark. It doesn't help your case when you openly shit on 25 years of the series history while doing so.
Oh, and...
Also, no Dark Priest option. Deal with it.
Who even started that? It's beyond lame.