I would say that the main things are a consistent positivity, spontaneity and a femininity that my wife lacks. My wife basically has PTSD from the darker times of our child's illness and can easily get back into a funk if even the sniffles arise. In fact I now experience an similar reaction when she starts to talk about health issues. One of our other children had an infection that caused her blood platelet counts to drop sightly, and my wife casually started talking about how is she was concerned that if she slides at softball she might bleed excessively or bruise a lot. I had the momentary sensation of nausea. I tried my best to stay even keeled and said "I can see the concern, but I think she'll be ok. Let's see what the blood tests show next week". But internally I was almost sick to my stomach. The platelet count was fine after treatment.
The feminity thing may be just because I "took a bite of the apple" by having an affair, I'll readily admit that, but let me explain. My wife is very self conscious about her looks, and behaves almost in an asexual way. My affair partner is strong, intelligent and though she has her own insecurities, confident in many settings. She knows what she has to offer physically. It was a huge eye-opener to have someone like that by my side, who was not an iota less strong or intelligent than anyone else but brought a feminity and a subtle deference to me, or better put, understood that her feminity was a complement to what I brought to the table as a man. It's funny, I'm very egalitarian in my words and beliefs, but that aspect of the relationship and the meaning in brought caught me off guard. I can say with high confidence this is not something my wife could ever achieve unless her self image and way of carrying herself changed dramatically.
And as an aside, my affair partner would love to be with me long term. But she's smart enough to know that my recent equivocating are a red flag and I'm sure she would have pause unless I totally divorced of my own accord. Then she may consider me an option once I'm free and clear and may have moved on in the interim.