Where to begin.....
My wife and I have been in the LS for 12 or so years. We have evolved greatly from where we started to where we are now, which is mostly solo play vs couples. Don't get me wrong, we very much enjoy couples, but we have a very difficult time finding that all important 4 way attraction and chemistry, and frankly couples that are on the same page. We have a couples profile and I have this SM profile.
I've had a few long term relationships (and relationship is the word to use) with couples that were just wonderful. Mutual trust, attraction, and oh such chemistry in the bedroom. Vanilla times to together were as common and LS time together. It's been some time now since I've been able to find that again, and that's what I look for.
I and not looking to notch the bed post. I need to have attraction both mentally and physically. Early on, I'd take all offers and quickly found that those encounters left me feeling flat and empty. That was a short lived approach.
While the responsibility to be kind, caring, and honest and work to develop a genuine rapport falls to all involved, I feel it is incumbent on the SM to understand the dynamics the couple is looking for, what the boundaries are, watch for a changing landscape, and to support their experience.