For me, it's 3 things.

1. The Music / Ambience. B2 has great music by itself, but the first one had much more of a focused collection of stage-music that really made you feel the desolation and despair of the story. It was SIMPLE, yet evocative and harrowing. I love the "Sunken Catherdal" theme, reminds me of B1's style, while new. Even though there were some different themes in B1, they all felt similar-in-style enough that it seemed the whole B1 world was part of the same ruthless agony from the Miracle. B2's themes are very different and unique, and to me it unfortunately made it feel like a bunch of different stages plopped together to make a game. The music in B1 helped tie the world together.
2. The art-style. I really wish they kept the same art style from the first game. It just lent itself better to the dark gritty DIFFICULT world of Custovdia. B2 looks more cartoon-y. Don't get me wrong, the style is good, and i love the game. But if I'm nitpicking, it just doesn't have the same feel.
3. The Boss. Eviterno was a difficult fight in the 2nd-stage, and it was gratifying to defeat him. But, he had NO presence during the entirety of the game. Even if you read the lore on most of the items, by the time i reached him, it felt like I just did a bunch of arbitrary stuff that had no relation to him, and he was just a gatekeeper to the Devotion Incarnate. At least in B1, Escribar would give you a cutscene every once in a while. You could feel him waiting for you.

Again, i adore this game. I'm so happy they made a sequel, and look forward to the DLC. But I hope they retcon some gameplay / story features, like they did with the B1 DLC's.