One common complaint I see across the many fandoms I've come to know since then—whether I dived in completely or just stuck in a toe—is that a canon pairing doesn't have enough “development”. And I want to take a moment to contest that complaint—not for any particular case, but as a general rule. That's not to say that I think such a complaint could never be valid. But I think it is a tired, overused complaint that often doesn't really take into account the full scope of what the author is trying to do.
The first problem with the complaint is that it doesn't take into account the role of romance in the story. If you're reading Twilight or watching Letters to Juliet your complaint about lack of development between a romantic pair would have more validity than if your media of choice is, say, The Lord of the Rings or Naruto. Romance clearly plays a larger role in some stories than in others; you can't expect an author to spend loads of time developing a romance when there's, for example, a war going on. Especially if the characters involved are supporting characters in a large cast. But even if one or both members of the pairing is a protagonist, only so much time can be devoted to the protagonist's romance.
The second problem is that “developing” a romantic subplot in a story where romance has a relatively small role requires tying it to the main plot, or at least to a larger plot line. Hogwarts doesn't have a school dance every year; instead, they have a special Yule Ball that is part of the Triwizard tournament and connects Harry's competitors in the tournament more closely to Harry and his friends. In HBP, the clues that Remus and Tonks like each other are hidden among general news from the war. This limits the amount of information that can be provided about each couple's interactions.
Often those who cry out for “development” will point to examples of stories in which the protagonist and his/her love interest—or else two co-protagonists—share some adventure and through it fall in love. That's all well and good as far as it goes. But it's unfair to expect every adventure story to take that route. This is where I, personally, appreciate a little touch of realism. In some stories, the protagonist pairs off with someone that they meet later in their adventure, or someone they left behind who didn't get to come on the adventure. Or perhaps two supporting characters who only interact a little bit on-screen or on-page while the adventure is going on pair off at some point. The fact is, these examples are much closer to the reality of how people meet and fall in love. The Speed method of romance is by far the exception rather than the rule in real life love stories. Most people meet and fall in love under far more mundane circumstances.
And such mundane circumstances are hard to tie to the main plot. A good example is the story of Arwen and Aragorn, which Tolkien chose to tell in an appendix because it didn't quite fit into the canon proper of LOTR. That's because their relationship was mainly established in quieter times, before the war of the ring began. The cinematic version found a way to work Aragorn and Arwen's relationship back into the main plot, but what worked for the film would not necessarily have worked for the already very dense storyline of the book.
The complaint of lack of “development” seems to set up a false dichotomy. Either the romance is fully integrated into the main plot, or the author is not “allowed” to pair those characters together. IMO, that is much too restrictive. I like romance. I want to see it happen in many different ways, not just the His Dark Materials model of love in the midst of struggle. I even enjoy having a pairing revealed as a “surprise twist” in the plot, as long as it's not involving a POV character, and as long as it doesn't “undo” another pairing that I enjoyed more. (And even then, sometimes I can forgive.) I think authors should be allowed to have a wide variety of options for including romantic subplots in their stories.
And now, some multi-fandom tribonds I came up with in the last day or so:
1. Faramir...Molly Weasley...Ron Weasley
2. Prince Zuko...Eowyn...Luke Skywalker