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.
I totally agree, I realy enjoy Moffat's storytelling, and I hope he does alot more Doctor who.
I just try to enjoy the show so the trying to make something out of a minute scene (stupid ducks in a duckless pond for one) seems a bit much for me. Or trying to come up with outlandish outcomes that storywise make absolutely no sense. I wasn't spoiled about the River storyline. I had started to wonder if that is where they were heading with the story. NOt sure when it started me on the path to that. But I was still very pleasently surprised and happy with the outcome.