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.
„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.