For example, take Mark and Lexie from Grey's Anatomy. There's no two ways on how they hooked up: Lexie shows up demanding sex and Mark goes with it (after initially saying no). And that's pretty much set in stone. And, I don't mean to imply that there's anything wrong with that. There isn't. Because that's what Mark does, and you expect it from him. But, there's that lingering question of, well, what if he hadn't slept with Lexie? What if he had said no and stuck to it? I think the result would be something like
citron_presse's Warm Than Warm (which I think was written before the episode aired). It opens entire doors as to WHY he didn't, which, IMO, would have added a tonne of depth to his character if they had kept the UST.
In terms of fanfiction, you can deviate that storyline and still write a fantastic fic. But, you are much more limited in expanding on characterization and having that characterization seem plausible in the canon world. I think that partly stems from RST relationships being more defined in all aspects of the relationship than UST ones. You can always speculate on what may or may not be there, but that's hard to do when you know for sure something's not there. In Stargate SG-1, you always saw only glimpses of how Jack handled Sam's disappearance; that sort of thing lets your imagination run amok with infinite variations on his characterization, particularly his thoughts during the situation, and because his reaction was left somewhat undefined you can look at pretty much any fic and say Yeah, that's what he was going through. It lets you have a much greater range of possible reactions for his character and no one can really say to you That's wrong. Unless, of course, you're extremely OOC, lol.
On the nonfiction side of things...
Then, there's the communication between characters. This includes everything from verbal to body language. For people who ship characters that have tremendous UST, body language is exploding with subtext. And, hello is never just hello. Which is part of the fun, if you ask me, because you have characters that can't say or do what they want to say or do, and you end up with these very intense and powerful emotional scenes where, for one moment, everything just spews out into the open and they'll say things that seem very innocuous on the surface when, really, they mean something else entirely.
Take for instance, in Law & Order: SVU, when Elliot says to Liv, I'd give you a kidney, its' just that much more exciting/painful (for shippers) to hear because you know they can't say I missed you or I forgive you and this offering of a kidney turns into something symbolic. Likewise, when Jack's and Sam's memories get erased and they have no recollection of each other, they are still drawn together, with Jack even acknowledging that he remember s feeling feelings for her.
I think the latter scene between any two characters would be great regardless of whether the two are actually together or not. BUT, it undoubtedly packs more of a punch when there's UST as opposed to RST. Because when you think about it, these two characters who have never gotten together, never said outright I love you, or even went on a casual outing (as far as canon goes), somehow they remember there used to be something there. They still feel a pull towards each other. That's how deep their feelings go. Now, if this were an RST situation, you would on some level expect them to remember, to be drawn together because of the memories they created and the Great Love they share. Otherwise, the love isn't so great, is it?
I know when Mark and Lexie eventually say their ILYs, it'll probably be a great AWW moment but I doubt it's something that's going to stick with me. I'll probably never obsess over their scenes the way I do over Jack/Sam scenes or over El/Liv. I probably sound like I'm picking on M/L, but that's totally not the case (intentionally, anyways). It's just they're the only RST couple I ship so I have nothing else to compare my UST ships to, lol.
Overall, while I love my RST ship (Mark/Lexie), I think UST wins hands down for me in any situation. And, yeah, I bitch and complain about El/Liv not getting together, but I immensely enjoy the UST. Generally speaking, I think UST scenes are far more emotionally powerful and character developing than anything I've seen with RST couples.
What do you like better, RST or UST? Why? Think I'm completely wrong in my analysis of either dynamic? I encourage all comments and discussion (especially from authors)! I enjoy talking about things like this, so don't hesitate to give your opinion below (i.e. help me procrastinate!) =)