Yes, let's get the obvious one out of the way first. While you have to feel bad for star Finn Jones - it can't be nice to have so much criticism flung at you online - it's hard to argue that Iron Fist isn't the weakest of Marvel and Netflix's five leading heroes. Some of that is to do with Jones simply not being as versatile a performer as some of his cast-mates, but mostly it's the fault of the writing.
The likes of Daredevil, Jessica Jones, and The Punisher all offer up a compelling central character who is easy to root for. Iron Fist, however, is a petulant, filthy rich man-child who likes to tell us how awesome he is ("I'm the immortal Iron Fist, protector of K'un-L'un, sworn enemy of the Hand!") far more than he actually proves it to us.
In The Defenders, the character marginally improves as his childlike optimism provides a decent contrast with the cynicism of his fellow heroes. Still, even that show got so bored of him he was incarcerated in some way for about half the series. Here's hoping Iron Fist season 2 can fix him further.