At the end of the day in today's world where people discuss and cogitate things online it's easy for people to come up with the right answer. People had formulated all sorts of possible solutions for who River was, and I think (yet again) some people miss the point. Some fans may have figured it out, but I reckon a great many people who watch and enjoy Who--but who don't spend every minute focusing on the minutiae of every episode-- were probably surprised. Plus as I recall the facts of Amy/River were posted online weeks before the episode anyway.
The essence of a good twist is that it should, in hindsight, be obvious, which Melody Pond/River Song was, as was Clara, but no people wanted Moffat to come up with something they didn't see coming, which is the worst kind of twist, like old fashioned crime stories where the detective solves the crime by suddenly bringing up evidence the reader was never aware of.