Transformation. The lovers as a catalyst for each other's growth/healing. In fairy tales, the transformation is actually a restoration, reclaiming your kingdom, your crown, your human shape. Dropping of a disguise. Your lover can see through it, and by seeing through it, bring you back to yourself. Transformation makes it a romance for me; I don't necessarily need a HEA.
Convincing obstacles. Obstacles make it a story; or it would be over too soon. Generally, a good balance of external and internal obstacles will be promising.
External obstacles, to show off the protagonists' capabilities and compatibility, when they work together to surmount them. They shouldn't seem too easily surmountable, because then the couple just won't seem terribly capable and compatible if they fret about them too much. I'm not at all a "love conquers all"-type, so I'm fairly easily convinced by the seriousness of an obstacle, even in a modern setting - I think class barriers, and disapproving families etc, actually make for pretty good obstacles still - but it's perhaps harder to show how that's still an obstacle nowadays, in spite of our characters' best efforts, and I definitely need to see some proper efforts here, so that I can buy into the struggle.
Internal obstacles, based on flaws to overcome, long buried hard-to face truths finally to be faced, counter-productive, no longer useful coping mechanism to grow out of, to make the characters multi-dimensional and dynamic, and provide the narrative necessity for the transformation that makes the romance. I can get very invested in couples who are sure of their feelings from the start, and only kept apart by external circumstances. One of my favourite novels is The Betrothed Lovers, by Manzoni - but I love that novel for the sweeping scope, the colourful cast of characters, the vivid evocation of the historic setting, and that tour-de-force chapter about making your way through the plague-ridden city to find your beloved - the romance isn't the main attraction here.
For the romance to be a main attraction for me, it needs to be a slow-burn, and at least one part of the couple needs to be in denial for a good while. I want some dramatic irony, I want to feel smug about my superior knowledge as a reader, I want to read about the protagonist huff and puff how they're totally unaffected and unsusceptible to a certain person's charms, and go, haha, just you wait.
My favourite obstacles are both external and internal - conflicts of loyality, contradictory goals, prior obligations, love vs duty. "The problems of three little people don't amount to a hill of beans in this crazy world"-Casablanca type stuff. It's probably my downfall as a writer and reader of romance, because I often end up not at all convinced that the lovers should prioritze their love, in the face of everything else going on in their world. There can be something profoundly anti-social about a no-holds-barred romance, and it's unsettling and powerful and can make for great literature, but I doesn't necessarily make me feel good, which is also something one might reasonably want from a romance. (*cough* Wuthering Heights *cough*. Great novel, shit romance.)
For the escapist fantasy and the HEA you really need to give your protagonists an option to choose both, love and duty, and it takes some real ingenuity to come up with a way to let them have their cake and eat it, after spending so many pages on making that seem impossible. It's a tricky balance to strike - if you try to mine too much tension and suspense out of setting your couple up for that choice, and then save them from that choice, that can feel like a cop-out. But if you don't, you won't have a HEA.
Also tricky, but ultimately easier to resolve in a satisfying manner, and therefore my second favourite obstacle in romance, is that the characters are, in a more or less immediately obvious manner, in their own sweet way, at least a tiny little bit idiots. Just complete, utter, staggering morosexuals.
Which brings me to another element I appreciate: Humour. Lots of people complain about romance heros and heroines too stupid to live, and I agree that a little stupidity goes a long way - there's a real possibility to overdo it. But it's a lot of fun if you do it right! The way to frame the required idiocy of your fools in love in a way that seems still attractive, is to surround them by people who are even more ridiculous.
Whether you use scathing, trenchant oberservations of human foibles like Jane Austen, or puns and goofy, silly slapstick like the Hong sisters, or often both like Shakespeare (and IMHO, also the Hong sisters, in their best efforts) - it will all accomplish the same useful effects: It makes your characters relatable and multi-dimensional (flawless characters are boring and who hasn't been a fool in love?), it provides a challenging, but also ultimately surmountable internal obstacle, it primes your readers for a bit of suspension of disbelief, creating a heightened sense of reality and signalling your intention to use artistic licence (surely, nobody would actually be that stupid; we are obviously not going for gritty realism here), which will make it easier to sell the escapist fantasy and the HEA.
Most importantly, humour makes people lower their guard. Which is why it's also very useful for tragedy (see, Joss Whedon, and, again Shakespeare, and also the Hong sisters, who are all very fond of using comic relief characters before the third acts). First make your audience laugh, and then hit them in the feels. If things are always and unrelentlingly dire from start to finish, we mentally gird our loins, preparing for the bad end and never get too invested in the first place. You have to catch people off-guard, get them invested without immediately noticing, while they still pretend not to take things too seriously after all.
Sure, humour can be used to deflect emotions, but precisely because it's often used like that, it can also be used to make people vulnerable - it's a perfect bait and switch! This second use of humour is what you want for romance. Dropping the disguise also means dropping the armor. At the end, the lovers have to be naked. Nobody gets to save face in a proper romance. At some point, everyone has to be humbled and mortified. Humour is the more gentle way to go about that.
But vulnerability is key, and that's my final requirement, it has to be equal opportunity. No lover must ever retain an upper-hand, they must both risk mortification and rejection, they must both put themselves at each other's mercy at some point. So the couple has to be evenly matched. It doesn't have be apparent - to the reader, to themselves - from the start. It's arguably often more interesting when it isn't. But by the end, it has to be clear that the lovers see eye to eye. Darcy has the status, but Lizzy has the social skills. Rochester has the status, but Jane has sound judgment, and the strong values, and the spine of steel. The challenge is to convince readers that these things are equally powerful. The power in status and money is easy to see; it takes good writing to show us the power in the other stuff. But it can't be mere lip-service - this is a place where you really need to use "show don't tell", you need to think of a plot suited to demonstrate that.
The more unequal the starting point, the more dramatic the reversal of fortunes you need to get to that place. I don't at all mind an asshole alpha hero. Rochester is such a trashfire of a man, I still love Jane Eyre. There's a let of fun in having some guy start out all high and mighty and then reducing him too rubble through the power of love. But you really can't pull any punches when it comes to the reducing to rubble part. Have him be rejected, abandoned, almost killed in an effort at redemption, maimed and permanently disabled, wailing to the heavens, come clean with his god. Have her build a content life without him, find friends, find success in her profession, become financially independent. And then let's talk again.
I don't at all mind an asshole alpha, but I feel that in modern romance they're often not nearly made to suffer enough.