In my experience, poly relationships seem to be much more fluid than mono ones. For instance, with a mono relationship, there are all these fairly concrete levels: dating, being exclusive, becoming a couple, maybe getting engaged, maybe marriage, maybe breaking up. And when you break up, it's over, because it has to be before you can move on to another relationship.
Since I've become poly, however, those levels are all much more undefined. There's a more natural ebb and flow to the relationships. With all my poly partners, there was never a break up, just a divergence of lives or wants or needs. I'm friends, close friends, with them all, because it was very natural to let them take their own path. Sad, when that path turned in a different direction than mine, but there was no sense of scarcity, or fear of being alone forever, or having to jump back into the dating pool - all those things that can be daunting if you're only allowed to have one love at a time. I think that when you only have one, it's harder to let someone go, and I was more likely to hold onto a relationship much longer than I should have than I am now.
Of course, this is only my experience, and I'm sure if I had ever experienced a betrayal or some other devastating situation, I might feel differently. But, for me, the feel of a break-up is vastly different as a poly person than a monogamist.