It's cold on The Wall. And damp. And boring. Sooo boring.
And a good writer can express to the reader how boring something is without making the reader bored. We know what boredom is. We don't like it. So don't make us experience it.
The third act gets a little better, but just when there's about to be something interesting... there isn't. And it's over.
I only finished the book out of stubbornness because it's not very long. (And I was reading a more entertaining book at the same time.) But even in its brevity, it was a struggle to get through.

Edit: I just saw this was longlisted for the Booker. What?!