What Most Ensures I Can't Put a Book Down?

Plot, right? No, characters. No, world building...
OK, so they should all, in a perfect reader's world, be a factor, but ultimately are they enough?

Recently, reviewing two novels back to back as it were, I found myself making a more direct comparison than usual. Both were carefully constructed, and would tick most boxes, but...
Novel 1: was Victorian era, with amoral vampires and a creepy human villain; nice twist there?
Novel 2: was Victorian era, Steampunk, with a mix of fae and human (Chinese and English) characters.
Both sound interesting, yes? I thought so.

Now as some of you may know, I collect vampire stories, always looking for new versions. And I'm very wary of Steampunk, which I feel too often degenerates into gimmicks. So without thinking I guess I had... expectations.
 So why did I love Novel 2, and wasn't hooked by Novel 1?

Because 2 had that extra, almost indefinable something: I cared about the characters.

Please note that doesn't mean I had to like them, but I did need to feel for them? Because otherwise - you've lost me, however clever the plot, the writing. Oh, I might skim through to see how it all ends but I won't be riveted and I very probably won't look to buy more of that writer's work either.

So for me the key question has become: do I care enough about what I'm reading??

[Note: I promise to add Novel 2 to my 5* reviews here, when I have a minute?]