Nicholas Paige (the editor and translator of a new edition of the book) argues that Zayde is the "last great French romance," and interesting because it is both a romance and a kind of pastiche of one; a book where all the standard tropes and techniques appear, but don't get used in their typical ways. I would perhaps add to this that the techniques, when used, come across as somewhat strained and tiresome. The mistaken identity issue is carried to such an extent that it almost seems like a parody, not to mention, largely unnecessary. As Paige points out, very little actually happens in much of the book. It's quite dull, really, until it picks up about 3/4 of the way in, in a flurry of agitation that feels mostly tiresome.
One thing I found intriguing was the way the book somewhat relied on a mysterious prophecy ("you will marry the man in this painting!"), wavered on its value ("perhaps they can convince her father it doesn't matter!"), made it come true ("you ARE him!" uh... spoiler alert, sorry), then explained the somewhat coincidental chain of events that brought it about ("it was all a ruse!"). Really odd, when you think about it. Undermining the otherworldly, then having it cash out (because maybe it could be kinda true!), then undermining it again. Wacky. Strangely wish-washy for a novel written in the 1660s.
Perhaps more intriguing though - are all romances like this? - is the way that the book almost reads like a philosophical thought experiment conducted to think through how love works. What is true love? What makes two people fall in love? How can jealousy be overcome? How can you ever truly know another person, and do you actually need to, to love them? The novel stages these extensive discussions on the nature of love then promptly acts out the ideas mentioned in them. People pause to tell stories from their past that contain nuggets of wisdom the characters need in order to reform (which they maddeningly don't seem to understand).
Actually, the main sub-plot of the novel, one could say, is on the problem of misunderstanding, or miscommunication. In this, it is perhaps an interesting early fictional work, in that there is a real problem about intelligibility and understanding. The characters are either deceived by other people, or deluded by their love, jealousy, or general emotions. Feelings completely overpower reason in this book, repeatedly. People constantly project their ideas onto others and take them as true. This is especially charming when a guy is falling in love with a woman who doesn't speak his language, and the novel is simultaneously trying to claim that this really is true love (as opposed to all the mistaken cases of love in the story) and illustrating how completely misguided he is in all of his views of her. Pretty thorny, but the novel hardly seems to notice.
Not a bad read, but not an especially exciting one either, I have to say.