We've read three trilogies this semester by three authors, two of which are very good at what they do. I've figured out a few criteria for being a major author:
-The author must clearly put a lot of effort into their work.
If the author doesn't care about what he's writing, there's no love in the writing of it.
-The author must make sense past its target audience.
The best stories have meaning for every reader, not just the people it's written for.
-The author must be able to follow the conventions of fantasy and, well English.
Come on. If you're trying to be a great writer, you need to know the genre you're writing.
-The author should do something clever with what he's working with.
The genre of fantasy offers a plethora of cliches. Doing something interesting with these cliches makes your writing seem that much cooler.
These four things, while not guaranteed to make you a good writer, certainly don't hurt.