So far (through the end of May 2013), did any of the DCnU (New 52) DC comics make references to events/characters/details/etc... that happened in works outside of DCnU line?
In other words, are there ANY cases when, in order to obtain background material for something explicitly referred to in DCnU comic, I would have to read a non-DCnU comic?
Even a single example would suffice for a "yes" answer.
Yes, they decided to keep the most important aspects of the characters' history. Also since Green lantern had just rebooted in '05 under Geoff, the new 52 "rebooted" by picking up exactly where the story had been, though like the other reboots Hal does get a few years younger.
http://www.newsarama.com/7812-harras-berganza-dcnu-will-keep-much-of-dc-history-intact.html
Newsarama had already found out from more than one creator that the rich past of the DCU isn't being completely erased, and apparently, big changes are being explained in story. As Batman writer Scott Snyder told Newsarama last week: "We're very respectful of history and continuity, especially for Batman, who has some of the best moments in comics history."
And
DC confirmed that stories like Blackest Night, Brightest Day, Identity Crisis, Death in the Family, and Killing Joke are still part of the DCU history. In fact, editors said, some events in those stories are specifically referenced in September.
http://en.wikipedia.org/wiki/Green_Lantern_(comic_book)#Volume_5_.282011-current.29
As a result, the new volume of Green Lantern continues directly from the events of War of the Green Lanterns, with Sinestro serving as a Green Lantern and Hal Jordan beginning the series powerless on Earth.
Now if you are an existing reader, you will find plenty of callbacks to either existing stories which transitioned into the New52 such as Green Lantern or Batman Incorporated, and may derive extra enjoyment from this knowledge, but new readers will not be required to go back unless they want to know more.
It is not necessary to know anything about the DC Universe to read their current Universe depiction and continuity. All previous works are consider apocrypha. While it may be nice to know some of the historical references, they are not required for new readers.
The goal of the New 52 Universe is to erase as much of the previous continuity's calcification in terms of story ideas, in terms of relationships between characters, in terms of removing the burden of history from the writers, allowing them a fresh start with the iconic characters of the DC Universe.
Ultimately the goal is to allow new readers who are unfamiliar with the characters beyond their iconic origins to have a brand new starting point. This has caused bad blood with long term fans, of course.
This is done wherever possible except where doing so invalidates something needed to maintain the current continuity. Does it happen? In a Universe as convoluted and as often rebooted as the DC Universe, it is likely mistakes happen but the goal is to avoid referring to any previous continuity, where possible.
Whenever DC rebuilds their universe either through multiversal crises (Crisis on Infinite Earth or Infinite Crisis) or through stories which correct smaller storytelling flaws in the universe (Zero Hour, Flashpoint), the goal is to correct, erase or retcon a difficulty in a character or location background that cannot be ignored any longer.
