Top of my list would be communication. You have to express what you want, where your boundaries are, how you're feeling. Equally, the ability to listen to partners and understand their needs is vital. ENM isn't always easy and there are sometimes difficult conversations.
I think you have to be quite self-aware. To recognise your emotions and understand that it's ok to feel jealousy or insecurity, that it's just an emotion, that your thoughts aren't necessarily truths. Yes, she might be thinner/prettier/funnier but that doesn't make her "better" or lessen your qualities.
Trust is a huge part of ENM. Contrary to popular belief, it's not having your cake and eating it. I'm in a primary relationship with DP and we're always respectful of each others' wishes. If he wanted me to stop seeing someone, we'd discuss the reasons but ultimately I'd prioritise him.
Finally, organisation. It's hard to coordinate multiple diaries. To remember who you've watched that Netflix series with! Especially if your partners have partners.
I think people are naturally monogamous or not. Many people would struggle to share. But I also believe society has taught us that we should seek only one person to be our confidante, our lover, our co-parent, our best friend.... I don't buy that anymore. I'd prefer to meet new people with no expectation of what our relationship might be. They might become a really good friend, a casual acquaintance, a passionate lover, a life partner... it's nice to feel that I'm open to all those possibilities. That I can surround myself with people I love, like and respect without labels or restrictions.