I watched all of both seasons of Daredevil, about three episodes of Jessica Jones, one episode of Luke Cage, and no episodes of Iron Fist (thanks to reviews on here), and I still watched and enjoyed the Defenders series. I guess I watched just enough of the other series to get the characters. Enough people are enthusiastic about the Jessica Jones and Luke Cage series that I have no problem stipulating they are good, they just weren't my thing, whether it was pace, tone, or what. It wasn't the acting, which was fine, and I enjoyed all the characters in the Defenders.
If you liked Iron Fist I don't see why you wouldn't want to watch the others, but if you absolutely couldn't, with a few episodes of Jessica Jones and a couple of Luke Cage, you could probably follow the Defenders. But might as well watch them all, The Defenders series isn't going anywhere.