Hrm...seeing as people seem to like my video (Glory of Love) and have stated it as an example (thanks by the way), perhaps I should chime in. Especially since I have a differeing opinion compared to the majority here.
My first piece of advice is a simple one, watch a lot of romance videos (good and "bad"). You need to do this to realize what it is you wanna do. What you think is "good." This will help you to realize what's "good" and so on (i.e. answer your own question

).
Since I see a lot of my favorite romance videos not being mentions, perhaps this is a good chance to advertise them:
-
Transcending Love - people keep talking about story. This video creates it's own original romance story using three separate shows. I was quite surprised no one has listed it in this thread already.
-
I'll Stand by You
-
In My Arms Again
-
My Fading Voice, Your Distant Hear -sadly not uploaded.
-
Through Time, Through Space
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A Thousand Miles
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Let Me Save You
-
Memories of Love and Conflict
I almost always watch these romance videos prior to making my own. They're wonderful videos and even if you don't like them, you'll at least see what people perceive as "good."
However a disclaimer, I don't at all mean to say you need to do what these people are doing. You should ultimately do what you wanna do, but it's always good to watch things to jump start your own thoughts.
Now onto my own feelings on what makes a good romance video. I see that the most often made comment/advice is story. I don't entirely disagree with this statement, but I also don't totally agree with it. Let me explain:
First off, most of the videos people have listed (Sappy Self-Indulgence, my own, Far and Away, etc.) do not have a coherent and concrete story. As Rosarch clarified, they have a *THEME*. That theme is obviously romantic in nature.
I think SSI is the best example of this. It's easily the most popular romance video on this site (and one of the best), yet contains no story. It's
just couples lip-syncing to each other and being lovey-dovey....right?
However, again, they do not follow a traditional story like say point A->point B with conflict sprinkled inbetween.
I know for a fact with my own video that I wasn't telling a story. I was more trying to communicate a feeling. I was trying to communicate the love and devotion two people have for each other. Yes I highlight some of the story elements and things they endure for each other, but ultimately the video is about communicating a feeling. Perhaps people say "story" since I go over some of the elements of the story and yes, there is a resolution, but I guess I have a different definition of story. Let me try and explain:
I guess by using an example, I think "Transcending Love," is the only AMV I listed above that is purely story driven. It has an actual plot. A beginning, conflict and an end. Again, I realize might disagreement with people saying story might be a misconception in terminology.
At the same time, don't get me wrong, a story can help a romance video considerably. My point is that it's not the main thing absolutely necessary. Obviously "Transcending Love" is a grat romance video that focuses on a story.
I also see that some people are making suggestions on music being light and soft. I can see how one could come to this conclusion, but I disagree. Yeah good music is needed for a good romance video, but good music is needed for almost any good video (regardless of genre) right?
Yeah some of the examples I provided above have slower and softer songs (in particular "In My Arms Again"), but others (like "A Thousand Miles" and "Memories of Love and Conflict") have songs that are a bit more dynamic than a traditional lovey-dovey song. My video, to a degree, is not really all that slow either. Yeah it's lovey-dovey and sappy as hell, but it's not slow or soft. The verses are slow and soft, but the chorus is dynamic (and in turn I use dynamic scenes).
So what's the point of this post? Did I just wanna argue that people were "wrong" and then not help you? Nah, I have my own definition of what is key to a good romance video and I'll share it with you now

.
It's emotion. Pure and simple. If a video doesn't contain emotions, it's obviously not a good romance video (which relies on...emotions.

). You could have a video with tons of story, a great slow song and it could still be a bad romance video if it doesn't resonate or touch someone's heart.
I guess that sounds ridiculously sappy and ghey (what did you expect from a romance amv creator

), but it's ultimately true. Earlier I talked about the examples people provided and how they lack story. By the same token, what do each of the videos have? Emotion.
Sappy Self-Indulgence may lack a story, but it's got emotions behind it. It's Sappy (:shock:) and a celebration of love. Viewers walk away and feel happy. It instills those emotions on the viewer. That's why it's a good romance video (at least to me).
Now I imagine people could argue that story helps this considerably since by seeing their struggles, we empathize with them and I can agree with that. However, I do not agree that the story portion needs to be the focus. It can be used in conjunction with emotions to ultimately make a good video. Make sense?
I guess I've sorta fallen into a trap since to a degree I'm saying you can't make a good romance video without injecting some sort of emotional content into it...and what if you don't have any emotional attachment to the song or series? Well that's a toughy I guess, but I have a confession to make.
When I made "Glory of Love," I didn't make it "for a girl" or anything sappy like that. I just made it because I thought the idea would work. I guess I'm gonna cop-out here, but I don't know how I injected any sort of emotional content into the video, so uhm...yeah not very helpful here
...actually, now that I think about it...I guess for me, the emtional aspect is that I adore this couple and their relationship and that's what carried the video and ultimately was my emotional connection to it *shrug*. OK maybe I'm reaching? >_>
And if you'll forgive me, I'd like to say something somewhat selfish. I have a romance video that I completed for Otakon sitting on my hard-drive waiting to be released. It's probably my most personal amv ever. That obviously means it has some emotions (personal) injected in it and considerably more than "Glory of Love." And yet, I still like "Glory of Love" more and I think people will feel the same way.
Perhaps that somewhat discredits my argument and I'm rambling, but I guess I'm curious to see the reception it gets when compared to "Glory of Love." More emotional ties and yet even the creator (me) doesn't like it as much. In any case, for those interested, keep your eyes peeled. I hope you enjoy it

. (/pimp)
At the same time, I may be sharing what *I* think good romance videos have, but ultimately that's just the opinion of one person. Each of us has our own opinions and ultimately you'll figure out what you like in certain romance video by yourself. Hopefully this discussion will help you to come up with what you like, but if you come to some other conclusion, more power to ya! And uhm, sorry about the long post, but I am (sadly) a romance editor by nature.
Good luck with your romance video!