I had the same reaction to the Mistborn books, and my interpretation is that Sanderson really struggles when the plot and characters get "wide". The first book was tight and focused; when the characters graduated to a larger stage everything - from the characters to the world to the plot - ended up "flat" and unrealistic. It's like he has a set budget for any given work, and the wider the focus the less of that budget any individual piece gets.

Which is a shame, because some of his work really is excellent.