Many passages are crafted with such care that they reach the level of poetry, which should be no surprise given that Pasternak seems to have had aspirations as a poet. (Is it just coincidence that the good doctor does so much of that kind of writing in the novel?) The short passages and lightly threaded vignettes characteristic of the novel are captivating. You get the initial sense that the novel is an underrated Russian classic. Ultimately, however, Pasternak doesn't quite reach the level of mastery of a Tolstoy, Dostoevsky and Turgenev, even though it aims for their kind of sweep and scope.
Doctor Zhivago is generally less coherent and cogent than any of the true classics of Russian literature. Many of the vignettes embedded in the book are beautiful, and as passages of near poetry, almost can standalone. Yet, as moving as much of the book is, its heft is insufficient at helping Pasternak push towards any master theme, focus or even plot, so the force is dissipated at the end. It is no accident that the book's conclusion and epilogue seem aimless.
What redeems the novel is the wonderful characters: Zhivago himself, of course, the brilliant doctor, with the dreamy poetic sensibility; the beautiful and bright Lara (who unfortunately ultimately fails as a character, though, because it's unclear what she is meant to be at the end; the contrast with Tonia could have been made better if Tonia had not been left so two-dimensional); the tactician Strelnikov, whose combination of military brilliance and ruthlessness enthralls us; the way he exits the novel at the end is unfortunately also a disappointment).