Something I've become very aware of, in my evolution as an author, is the difference between being a great writer and a great storyteller. I am a skilled writer, but I am not a skilled storyteller. Storytelling is my biggest challenge - I can write strong scenes, but I struggle immensely with finding an engaging and sustainably interesting story arc.
Most of the books I judge as being sub-par really are sub-par when it comes to the writing. But where they're strong - and the reason they got published - is in the storytelling. Fifty Shades of Gray is horribly written. But it's a compelling narrative that millions of people find impossible to put down. Same with pretty much every book by Dan Brown. I wrinkle my nose constantly through his books, but I still read them because the stories are great, even if he misuses intransitive verbs.
Some books have strong writing and storytelling. Some have just one. It's just different ways of being good at the craft.
This is a very good point. I have reasonably decent language skills, but I can't find story for nuthin'. And I've read a number of novels lately by authors with less than stellar writing chops—but the storyline kept me reading.