In my case my husband Alec is not in other relationships and possibly mono. I've been thinking a lot of this, since I do want him to get something out of all this as well. I want him to be as happy as I am.
And, there do seem to be quite a few things. Alec and my gf Mya are developing a friendship of their own, which has been really nice to see. Also, we are both getting to know Mya's husband, and there seems to be friendship potential there as well, all of us get along well.
But I think there are some things within our marriage, which have been positively affected by poly. Well, one concrete example is the fact that NRE has woken up my hormones and we are having more sex with Alec, as well. The hormonal influence will very likely be only temporary, but more sex has been one factor (among many, of which some not poly-related) enabling us to talk constructively about our sex life, since the issue of frequency wasn't acute. And the fact that we've been able to open conversation is a really important thing. This goes for other areas of life as well: we are communicating a lot more since becoming poly, which really enriches our relationship and both of our lives.
Additionally, there are the changes in me. Well, firstly, I used to be happy but now I'm even happier. And he likes to see me happy and glowing and it makes him happy, too.
Also, I've made a conscious decision to always be present in whatever I'm doing at the moment. And that includes time spent with him. When I'm with him, I focus on Alec, and concentrate on seeing him as he is and everything I love about him, and listening to him and finding out what he is like. I don't know if that sounds sort of obvious OR all strange. But I feel it is a different way of relating than I had before. Maybe it's something that easily happens in long relationships, but we spent a lot of time just hanging out without really connecting. Now when I'm with him, I'm present, and concentrating on what we're doing together, even if it's just watching TV. This makes a huge, positive difference to me, and I'm sure it makes a big difference for him as well.