I think it's hard to narrow it down to just a few short things but I'll touch on some that come to mind. Having similar values was important to me. Talking about the controversial or harder subjects and knowing that you can be on the same page with making decisions. Respect. Knowing that at all times we can treat one another equally and with consideration for our feelings. Stability. Knowing that we're at the same place in our lives and we have the same end goals. No alcoholism, drug use, and so on.
Having someone be able to understand you and your life experiences. I didn't realize how much it would mean to be with someone whom had a hard childhood like I did until I was. The solace it brings to have that person fully understand and not judge is irreplaceable.
Then just the feeling of connection and something individual about the person you meet. Loving their quirks and personality. Enjoying their company the most.