One thing that makes really, really good writing. Specifically in romantic situations.

If you're writing a story and it comes to a romantic situation, it can be really hard to pull off. Sometimes I find the most effective romance in writing is when things have an incredibly little amount of detail, and most things are left to the imagination. Using metaphors and letting your words sort of float around what's happening, and really describing a character's feelings rather than saying exactly what's happening can make an intimate scene perfect. Though good writing is always "show, don't tell", but I think when you push that even more when writing romance, it's even more effective.

I've seen so much fanfiction written well this way. Face it, when you read fanfiction, you're really not interested in the plot when you look for something good. You're there for the ship, the pairing. That's why FanFiction.net has character A and B filters. There are amazing writers out there that write fanfic, and are just so so good at writing intimate scenes.

I've seen it a few times, and it's this one sentence they write and it just sticks in your mind. Dare I say it, but I've been suddenly turned on with a single sentence because something about it resonates. I just sort of sit there and go, "Oh" and there's that kind of, whew, did it just get hot in here? One damn sentence. It's amazing. It's always subtle, too. Describing a certain touch, the way a partner's breath feels against their neck, or some sort of tingling, tactile sensation.

Seriously, I've even been telling these authors this, because they need to know how good their writing is.

I don't think there's really a how-to guide on how to do this. But I always remember the sentences.