This is as hard a question to answer as for any other genre. For me, a good novel has characters I care about, so their problems and goals become important to me. They also have pacing that is varied, so I'm turning pages to see what happens next but also getting enough "down" time with the characters to become invested in their lives and world. I prefer a leaner, tighter style of writing, but what is most important to me is the voice that comes through. Generally, I like to feel the characters' voices in the narrative, though good omniscient narrators can pull me in too.
As for the romance component, I like there to be chemistry between the characters that makes sense to me, as well as a plot that goes beyond just the romance. I guess what I mean here is I need to feel why these people are attracted to one another and I need to see and feel real-life goals that go beyond the relationship. It tends to keep me turning pages when the real-life goals are in conflict with the growing attraction in some way, and I want to find out how these people find a way to satisfy both or to alter their goals so they can be happy together without either one giving up who they are.
I was frankly bored with a romance I read a while back, and it's because I realized about a third of the way through that nothing was happening except for these two characters (a convalescent and her doctor) taking long walks together and becoming attracted to each other. Neither seemed to have any other goals or wishes. They were both rather dull people. Maybe this is a realistic portrayal of how romances often happen irl, but it doesn't make for an exciting read imo. I like the developing romance to be interwoven with stakes of some other kind as well, whether they be secrets, or plots, or intrigue, or scandal, or adventure.
I like a fairly high heat level in romances. I don't like it when the narrative closes the bedroom door in my face at the critical moment or fades to black. That's a personal thing, though, because romance novels fall along an entire spectrum in terms of explicitness.
I don't much care for classic "alpha males" who know what they want and will not take no for an answer (I can't get past how stalkerish those plots feel), and I frankly dislike rape that turns into love. Fortunately, there are more heroic subtypes to choose from these days (and guys really don't fall into the category of either being either classic alphas or milquetoast), and some authors seem to specialize in unconventional dynamics, even in historical romance. I also like historical romance novels that take into account real events and situations from their time. I'm not as much a stickler for the characters being average representations of the prejudices and attitudes of their times (though I dislike people who feel too much like modern people in fancy dress). I like abolitionists, suffragettes, people of color, LGBTQ couples and others who are underdogs dealing with a biased society.
Also, actual historical touches add a dimension of believability. For instance, I recently read a novel set in London in the 1873, and the author interwove the nasty smog event that hit in early December of that year into the plot. So often, romances set in Victorian London ignore the realities of the setting, or make only a passing reference to the discomforts of living in that era.