A good story is icing on the cake, it is cool to look at and if done well can be impressive as hell. But at the end of the day, the quality of a book is measured by it's mushy soft interior and the flavor of the creamy filling... Not the icing by itself.
It is worth noting first that those things carry different value depending on the media. When we are dealing with comic books, operas or tv shows, there are many more moving parts to it, those are instances where the writing is actually the icing and the visuals and presentation are the filling... For instance I care very little for the plot of naruto, but the fights look impressively detailed and well animated, and just because I want to catch EVEN MORE FLAK, JOJO is an example of how an art carries the whole manga on it's shoulders with a very... eh... plot.
On a novel however, these things are entwineed, the quality of writing IS BOTH THE FILLING AND THE ICING, a simple cake that takes a lot to balance properly.
Here is an easy way to show you... Pick you favorite book, any one.... Now remove pretty much every character stylization, take all scenery descriptions and replace them with their most basic, and write it in the most accurate formal language you can... And now, you know what makes your favorite book interesting right?
A good story is just an idea, and ideas without execution are pretty damn useless in the grand scheme of things... Not saying you cannot have a stellar epiphany of the greatest story every told, but for a novel to suceed you to have the words to MAKE IT stellar.
Now what makes for 'writting skill' varies...
Writing is an art form there are many things you can learn about it and improve, a movie maker would learn 'shot composition' while you as a novelist learn 'scening', a traditional artist learns 'anatomy' like you would learn how to describe movement.
Generally speaking, descriptions are a pretty damn good starting point for any writer, trying to narrate the same scene multiple times using different words to stimulate different senses and invoke different feelings on the reader.
Next i'd say story structure, learn to create the 'Rube goldberg' machine that connects your plot together. Wether you want to plan the muder from the start and then write around it, or you want to just find out along your characters how the muder took place is what makes of breaks a plot.
The flair added in to make it better, it is the thing we call characterization, but do not underestimate the value of it, characterization is what keeps characters consistant and their personas matching the world they live in... Remember it is fantasy for us but real for them, so sure you think dragons are cool, but if they burn down the MCs village and he is just fine with them, that's sort of... Off...
Finally stylization, learn different vocabularies, regionalism, some psychology might also help, and try and make things more unique, like a painter carefully selecting each color on the pallet thinking ahead to carefully match them.
Other details are exclusively up to you, but remember, each part of this process ADDS ON to the previous, none of these is enough to carry a story by themselves, so the more steps you add the more you have to keep track off.