There are many different kinds of love stories and everyone has their own preferences for what kind they want to see. Some people like to see an oil vs water situation that gets dragged kicking and screaming into a romance. Example: Katara and Zuko from Avatar: The Last Air Bender were originally meant to end up together but the creators decided not to go through with it last minute. I personally enjoyed watching it play out, seeing the characters realize the other was human too, and was disappointed by the ending. My mother didn't follow the series but saw the finale and was confused: "But he was supposed to end up with that other girl!"
Some people like seeing the passionate love-at-first-sight types try to woo a less-than-interested (if not oblivious) target. Example: School Rumble. Each character had his or her own thing going on and it was fun to watch them try and fail and stress out over it all.
Other people like to see a power struggle of sorts. Sometimes it's done through blackmail, other times by social status, sometimes by choice, but it's only interesting if that power relationship is reversed at some point. Once again, gonna bring up an Avatar reference: Varrick and Zhu Li.
Others still like to see bonds formed across unusual obstacles. Pochi Kuro http://www.mangahere.co/manga/pochi_kuro/ just got started but I find it adorable.
Personally, I like to see sexual tension, acts of trust, making each other laugh, cry, and blow up in anger, admiration, jealousy, possessiveness, but above all RESPECT.
Things to avoid would include shallow romances like Romeo and Juliet. Romeo was whining about some other girl not wanting to marry him right before he sees Juliet at a party he was crashing. I don't think they even had a conversation before he sneaked into her backyard and spied on her in her nightgown. They reminded me of high school kids making out way too hard between classes at school.
Another thing to avoid would be a lack of self respect. If a boy calls a girl ugly, she should want nothing to do with him. She shouldn't start crushing on him. He's a dick and she should be able to see that. An exception can be made, for instance, if the girl enjoys being put down. Like she has a fetish for it or something and it weirds the guy out. Even that could be fun to watch. I'm just so tired of reading through manga and seeing girls fall for awful guys just because they're cute.
One last thing, regarding sexual tension: why is it so rare to see girls actually lusting after a guy the way guys are shown lusting after girls? Not thinking "oh he's so cool!" with stars in her eyes but biting her lip and thinking "DAT ASS." All they ever seem to do is blush. How about some bedroom eyes? You know how women often complain about street harassment (which is absolutely terrible and you should totally call people out on it when you witness it)? I've heard men complain about the opposite: they feel like nobody finds them attractive simply because they never get that kind of attention. Ever. Nobody goes up to a guy and says "What's cookin', good-lookin'?" Even in those harem anime that are so popular, the girls never fall for the guy because they find him physically attractive. It's usually something he says, or the fact that he protects them or something. I'd really like to see more female lust from a woman's perspective.