Why would DC, who has been so skittish about pulling the trigger on the Justice League movie, use the New 52 as a blueprint instead of the 50 years that preceded it?

Because New 52 is more up do date, and both old and new readers are familiar with it.

How is New 52 "more up to date"?

I think it will be it's own thing for the most part. Other then the lack of red shorts I didn't see any connections between Man of Steel and the New 52. There doesn't seem to be any indication that Batman will be based off the New 52, but for the most part with him that doesn't matter. We don't really know where they're going with Wonder Woman yet but with the casting of Gal Gadot I've got a feeling she'll be closer the the original Wonder Woman then the Xena knock-off in the comics now.

The only real similarity is that we'll probably be getting Cyborg as a founding member of the Justice League instead of Martian Manhunter. Of course that's all speculation, but I actually think that's a good move on their part. Manhunter is just a hard character to introduce without of feeling forced or eating up too much plot. Cyborg of the other hand is very easy to introduce, and plus it's a little more color for the team.

lol.

the only real change is Cyborg for MMH. Cyborg is a lot more film friendly then MMH. No one can really deny that.

Because that's the New Continuity.

I hope we see MMH by the second JL movie, but I understand why they would skip him right now. It would take up too much plot time to explain why he's popping over from Mars just now, and I doubt they want to spend that on a more minor character. Cyborg is easy to bring in. Just have Victor Stone hurt during one of the fights in BvS then a little teaser clip at the end with him having a robot body. I guess with MMH they could say that he was already on Earth laying low, or maybe being held at Area 51 (captured as an adult, and not grow from a petri-dish like David Goyer wanted), but bringing in Cyborg still seams easier. With the time they save by cutting out MMH maybe they could squeeze in a second female character. That would make the line-up different from the New 52.