The difference between both is very subtle.
sich etwas/jemanden anschauen
is more like to regard
. There is an aim at looking, for example finding mistakes, evaluating, considering about.
etwas/jemanden anschauen
is more like to look at
. It need not have an aim.
Examples:
„Ich schaue den See an.“ Nothing about it, just relaxing.
„Ich schaue mir den See an.“ Let's find a good spot for fishing.
Unsure side mark: I've got a feeling, that this subtle difference might be overriden, if the sentence gets to complex by other pronous and verbs. Then, the nonreflexive case is always used. But I can't find a good example for it.