"The Secret Life of Walter Mitty" is an unevenly elaborate picture that combines brilliant fancy and crowd-pleasing mediocrity and a couple of things in between. In trying to weave together ambitious introspection, artsy humor and blockbuster charm, it turns out as a whimsical flick that plays heavy on the uplift, but gets a bit vague on its vision. This movie actually aims high that brilliance would apparently pop up every now and then. It doesn't make it to the top,