The upcoming Batman v Superman movie was delayed by close to a year. The reason quoted in wiki is - "time to realize fully their vision, given the complex visual nature of the story".
Does anyone know of a more detailed reason?
There are actually two questions here - What caused the film's initial slippage and why did they decide to bump the film by almost a year.
We don't know and the studio aren't admitting anything other than issuing a bland statement about wanting to "fully realize" the film's "complex" nature. Reading between the lines, I think we can assume that the film was overly complex and needed a hell of a lot of editing in order to make it watchable by a mass audience. This is DC's tentpole film and potentially the start of a franchise worth 10-15 billion dollars. My personal guess is that that's the sort of thing you don't want to muck up.
This one's slightly easier to answer. This article in Forbes gives a good indication of why they bumped it to the prime April/May/July 2016 slot instead of risking an August 2015 release. Since there have been no rumours of additional reshooting or recasting, this strongly implies suggests that the film may have only missed its release slot by a week or two but then got bumped by the studio simply to hit the new tentpole release slot they already had booked.
The biggest grossing film of all-time in August is The Sixth Sense in 1999 with $673m. After that, it’s Rise of the Planet of the Apes with $481m. The biggest opening weekend for August is The Bourne Ultimatum, with $69m, which is about what The Dark Knight made on its first day ($67m). There have been exactly four movies (Sixth Sense, Rise of the Planet of the Apes, The Bourne Ultimatum, and Signs) that have opened in August that have made over $400m worldwide. Opening in August would mean that Man Of Steel 2: Justice League Unusual would basically have to break any and all August box office records to achieve what would be a desirable ($750m-$1b) result. Marvel may be willing to risk it with their non-essential wild card property Guardians of the Galaxy this August, but no way Warner Bros. will risk such a result with their crown jewel icons.