First and foremost, if there is a chemistry, then no explanation is needed. Everything goes smooth without words. Those types of sessions are rare. But just because there might not be perfect chemistry doesn't mean it can't be a really good or "great" session.
Communication is important. Intuition, especially in the top, is super important. The submissive gives you their submission and trust and that's responsibility that shouldn't be taken lightly or for granted.
In most of my private affairs I've always ,for the most part, had to be in the dominant role. With partners who were kinky, I was in charge, but always in charge keeping an eye for the comfortablity and pleasure of who I was with. Communicating beforehand that if anything goes anywhere that you don't want it to go, speak up, it's cool. Keep your finger on the pulse and know when to "switch gears". Switching Gears is an expression I read used by an old poker player to describe flexibility and not being locked in to one idea. It applies to poker, applies to life too. So if you're on the top the onus is on you really to make sure everything is cool, be observant and make sure the person your with is having a good time.
As a sub, which is the role I'm always in at the Fortress, I have to communicate what I like/dislike. Try and be open minded, polite, kind, fun loving, respectful and trusting. Sometimes the trusting part is not always easy.
That's what makes a great session, at least for me.