People love to say, "Sex is like pizza. Even if it's done bad, it's still good." And then everyone laughs/applauds/retweets/whatever because people love to remind those around them that they love sex and pizza so much. Personally, I've fucked some real garbage-ass pizza, but the point stands: consensual sex is pretty much always good, because you're having sex. But there are some things that just make it that much better. Here's what it takes to go from "eh" to "AH!".
1. Chemistry. Okay, I hear you: "duh." But this is arguably the most important aspect of great sex. You've got to be into it. You've got to want it bad enough. Sex without chemistry is like eating a bunch of pieces of white bread for lunch: you're just doing it to satiate a physiological response, but the act isn't exactly pleasurable.
2. Trying not to feel self-conscious. It's only awkward if you let it be awkward. And although the most intimate possible experience two (or more) people can have together is a potential awkward minefield, anyone who can roll with the punches and recover from A) strange bodily sounds, and B) strange bodily fluids, is the type of person you want to have sex with.
3. Really feeling like your partner cares about your pleasure. You can tell when your partner isn't into it. If they act like they aren't getting anything out of it, that's an immediate boner-killer. But if you're both enthusiastic and legitimately want to see the other person get off, that alone can make for great sex, regardless of your technique or experience.
4. Foreplay. For some people, it's more about the journey more than the destination. Typically, that journey involves a detour into oral sex town. Plus, the longer you make out for, the more anticipation, and a slow build to sex is like a volcano erupting, except if lava was something awesome that didn't kill you instead of lava. I guess in this simile, the lava is cum.
5. Discovering something new. This won't happen every time you have sex, but whenever you discover something new you're into, that's great sex. Whether you try it together, or your partner introduces you to scarves and blindfolds, learning "Hey, this turns me on" is awesome.
6. A partner who gets them hot. No, this is not the same as a "hot partner." This should go without saying, and yet... it's important to have a partner you're into physically, not just mentally. You need both! Sometimes it's nice just to grab someone who seriously turns you on and not really worry about whether or not you agree on parenting strategies.
7. Raw animal lust. Instead of embracing the fun, awkward, messy side of sex, it's like your bodies are two machines designed explicitly for boning each other. It's the kind of sex that feels like it does in the movies, all photogenic and beautiful. The kind of sex where if someone walked in on you two having sex, it'd make that person question everything they knew about sex. They'd wonder if they'd been doing it all wrong.
8. Natural communication. Whether you just met or you've been having sex for years, having an immediate understanding of what the other person wants is huge. It's what wins teams titles and it's what keeps sexual partners from having to stop sex to explain, "here, move you leg like this... no, like this... this... just put it here." Of course, sometimes those explainers are necessary, and you should always be explicit about what you want, but the best sex feels spontaneous and natural.