To some extent it is, in supergee 's indispensible word, a homofilm, best approached as a separate work that happens to share the title with a very different novel. Like Kubrick's The Shining, which I started out disliking but have come to appreciate as a homofilm, it paradoxically includes unchanged (or majorly intact) characters, a selected subset of events that are the same, and even exact bits of dialogue but still manages to change much of the focus, major aspects of the the underlying message/philosophy, and even in some ways genre.
The novel Winter's Tale is indeed a love story (actually, at least two major ones), but it's a lot more. The movie doesn't even introduce the entire basis of the ending of the novel, the building of a supernatural bridge from the Battery in NYC to Heaven or something like it. (That isn't a spoiler, though who builds it, how, and whether it works would be.) In the novel, Peter Lake's love for Beverly Penn is about 1/3, but all of it is a paean to New York city (& the upstate refuge of a wealthy Manhattanite); in the movie, the love determines the entire narration, and only about half of the movie is as NYC-centric as I'd hoped.
Other parts of the novel are alluded to but not developed, which some viewers who haven't read the book find confusing. For instance, in the novel the Baymen of Bayonne Marsh, who raise Peter Lake to age 12, have their own fascinating and bizarre society; they are represented in the movie only by brief conference between Peter Lake and a single enigmatic man, who seemed to me coded as Wise and Exotic Non-White (I thought Native America, W. thought Asian).
Above all, the philosophy is changed. The novel does present some kind of cosmic game or battle underlying worldly events, but it's nowhere near as clear and simple as the battle between good and evil in the movie. To me, this is the change that most makes the film feel so different. In the novel, the effects of the supernatural and earthly realms on each other, and parallels between the two, are subtle and complex--even confusing to a lot of readers, but the exact opposite of simplistic. Simplifying this for the movie was probably inevitable, and I felt the movie does a good job with what it (at variance with the book) set out to do. Changes in Pearly Soames' nature and associates, for instance, often seem like something Helprin might have worked into his approach or have the same kind of magical feel as some elements in the novel.
As I expected, what the movie does choose to do the same as the book is done very well; and as I expected, this is mostly presenting visuals. The horse is wonderful! (I thought he had his own "cast" entry on the movie website, but I can't find it now.) The indoor and outdoor scenes are rich in light--which is a central motif in both the novel and the book--and seemed to me to have just enough lens flare and other romanticization. The use of color and monochrome is excellent. Fans of the novel will find some of the key images, such as the trapdoor in the sky of Grand Central Station, the baby in the boat, Beverly's rooftop tents, Central Park as a winter fantasia, or Peter Lake and Beverly dancing--even if the plot around such scenes has sometimes changed. Brooklyn Bridge is used well visually.
I liked the acting, too. In the few reviews I've seen, Russel Crowe as Pearly Soames has been faulted for growling and a lack of facial range, but I felt he captured the character fairly well. Most of all, I felt the acting of Colin Farrell and Jessica Brown Findlay made the core love believable, without which the whole movie would have been ridiculous. In fact, the ease and speed with which appropriate partners fall in love seems to me a problem in the novel--I think we're supposed to accept it just as we are supposed to just accept the (other?) supernatural elements, but I just don't apply the same standards--but the looks and tones of voice carried it for me in the movie.
Of course, I can't say if the movie would totally make sense to someone who hadn't read the book. W. (who has not) weighed in positively about one scene being clear, but I want to discuss that more with him. (Comments, please?) At least I can say that the movie does make sense as well as being--and it is--very beautiful.
Status: Off to work; should have been preparing to teach the first part of the novel to a student today!