I agree with HollyShamrock. My philosophy is if the story can engage the reader emotionally first, so they can relate to, and care about the characters, then it enhances the erotic buildup you're trying to achieve. Length really doesn't matter when someone is enjoying it. Personally, even if a story isn't great grammatically, or the writing is unpolished, as long as I can feel some emotion from and for the characters, I'll enjoy it no matter what if the plot grabs me and I want to know how it ends. However, if it's full of details that really don't belong or add to the story in any way, i.e., meaningless fillers/dialogue, then I'll get bored quite quickly and be more critical no matter how good the sex scenes might be later on.
That said, it helps to "know thy audience" if you are writing for a particular group rather than for yourself. From my short time here on Lush so far, it seems like people either gravitate to down and dirty sex without too much info about the characters/situation, while others prefer stories like I do. It's not easy to please both camps, so I believe writers should just write what THEY like and can get passionate about, because that's where I believe the best writing comes from. :-)