He's good at what he does. He's got more ideas for Fantasy series than many writers seem to have in years. He can spin a very decent tale, it's not his storytelling that's deficient. He is also a very "visual" writer, and he's got a sense for the "modern pace" mimicking that of Hollywood or American TV, that people growing up on video games and such find very appealing (but that put off a lot of people more into more classic forms of novels)

He appeals a great deal to people more into Fantasy entertainment than, say, into more adult drama à la Martin or extremely involved and complex Fantasy à la Erikson. But he's got enough depth going for his books to be more than fluff - his world building his gimmicky to the max, and full of "cool stuff", but it's still very well done in its style (better than most I know working along similar lines).

Sanderson's works, writing wise, aren't far removed from series like Dragonlance or the SW novels. It's not much better than that. What sets him apart is that he's able to spin stories far more original and imaginative than that while writing with much the same literary skills (and similar speed). His books are very entertaining, with fun of pretty cartoonish characters but usually pretty "cool". Yoda, Han Solo, Vader cool.

He was the good choice for WOT because he had such a big head start with his knowledge of the material. He clearly wasn't the good choice if Harriet had been looking for someone able to write more seemlessly like Jordan, or at least stay much closer to his writing than this. But it doesn't seem to have been Harriet's main priority, or she would have gotten someone like Sanderson to draft and put together the book then would have hired a professional ghostwriter to "Jordanize" it all.

Few from the new generation write like Jordan (or even like Martin who has a slightly more modern approach). It's more than just prose (though Jordan's is also fairly old fashioned, so is Martin's, and even Erikcson to an extent.. though they're all very different styles of old fashioned writing), it's also pace, style, imagery, references etc. Jordan you can compare to old writers à la L'Amour or the writers of serials like Dumas. Sanderson is mostly comparable to Hollywood writers, or writers like Kevin J. Anderson, who I believe his one of his friends.

I'm not suprised of Sanderson's success. I don't think much of him as a novelist, mostly because I really get the feeling he's too careless with the craft itself, writes too fast and publish his books without having spent nearly enough time and effort polishing them up. But I still think much more highly of him as a storyteller and worldbuilder, and he's much in tune with current tastes of younger readers.

Personally I think he missed his calling, because he's got one of the best imaginations on the market and Hollywood that's starved for such could really, really have used someone like him. If I was a network exec looking for a writer to whom to give the task of creating for TV a whole Fantasy universe and create stories and characters for it (rather than buying rights and adapting a novel or series), I'd hire Sanderson anytime. Mistborn would have made a very cool TV show. As a novelist he writes highly entertaining and imaginative stories, and for many that's enough to adore it. For others like me it doesn't feel like Sanderson's work has much appeal as novels. The writing itself is too deficient, and even on the whole they often give the feeling of being a bit underdevelopped, unfinished novels. His finished work often reads like other writer's drafts. But they're very appealing to those without the patience to bear with the usual, more classic trappings of the novel form, like descriptions, inner thoughts etc.