The most important thing about a story is that it is enjoyable. That means the story, the characters, and plot, and the imagery all need to be compelling. However... if you can not portray the story well, you get lost in bad grammar and spelling errors, and fall out of the story. The idea is that if you are truly engaged by a good story or book, you don't think "I'm reading a book", you think "what's going to happen next?" or something like that. Once you start to get lost in bad grammar and spelling, you can not only get confused as to what is happening, but you start to become painfully aware that you are reading a book... a poorly written book. No matter how amazing the imagery and the plot and the characters are, if you can't portray it well, the story will fail.Good grammar and spelling allow the words to disappear and ensure that what elements are there rise to the front of the reader's mind unimpeded
So I say YES, you need to have a great story, engaging plot, fun characters, and amazing visuals. BUT you must ALSO have the basic principles of English writing, grammar and spelling, down in order to really present the story well, to prevent confusion in the reader, and to show quality and not just ideas.
I am always trying to concentrate on a good story, good characters, and coming up with visuals I like, and try to write a good story out of that. I always check my grammar and spelling to try to prevent people getting distracted by them, and to get lost in the story and not the words.