Short Answer: Crisis On Infinite Earths.

Long Answer (you KNEW there had to be a long answer, didn't ya?):

Well, the transition was complete by 1986, post-Crisis and after Shooter had chased away most of the 1970s talent from Marvel to DC, but there were a few "false starts" in the 1970s.

First and most obvious was the Kirby Fourth World operation, which was neck-deep in continuity, and which continuity spilled over into other titles in the Superman family at least, such as Morgan Edge and Darkseid appearing in Lois Lane and Superman titles. Around the same time, the Superman title itself was being reworked, with Superman being depowered and Kryptonite being removed, under the auspices of Denny O'Neil. Superman from approximately issues 233 to 241 or so provide the best representation of this experiment, which sadly never really lasted. The other Superman titles, Action and World's Finest, stayed more-or-less solidly in the Silver Age mode.

Similarly, most of the mainstream superhero strips which survived past 1969 -- Flash, Justice League, Aquaman, Teen Titans, Wonder Woman -- didn't learn anything from Marvel's success. The two exceptions were Green Lantern/Green Arrow, and Batman -- and again, neither of those experiments lasted long, with GL/GA being cancelled within 2 years, and Batman reverting to late-sixties type until Englehart and Rogers came on board in 1977.

The big saving grace of early seventies DC was their mystery line: House Of Mystery, Unexpected, Ghosts, and the like. And of course, none of that was in the Marvel mold whatsoever.

Something else from the early to mid seventies to check out is the Spectre strip in Adventure, by Michael Fleischer and Jim aparo. These stories are collected in thr TPB Wrath Of The Spectre, and are of an equal or superior quality to most of the contemporary Marvel strips.

Gerry Conway defected from Marvel to DC some time in 1975 (although he spent a few years writing for just the mystery titles), and almost immediately set about trying to institute some Marvel-style continuity in the new strips he was given, Secret Society Of Super-Villains, All-Star Comics, Justice League, Freedom Fighters, Hercules Unbound, Metal Men, and Kamandi, among others. Yes, Gerry was a workhorse. Gerry moved over to some of the other mainstream characters in 1977, writing Superman and wonder Woman, but with a tight editorial leash, and then introduced his most lasting DC creation Firestorm just a few months before the Implosion. Much of Conway's work for DC in this two-year period is as enjoyable as his Marvel work, but after the Implosion Conway became just another DC writer.

The next significant Marvel creator to join the DC bandwagon was Marv Wolfman in 1980. Wolfman's ground-breaking work on New Teen Titans (and, of course, Crisis On Infinite Earths) is the most notable, and then in 1982 he was joined by his old partner-in-bloodsucking Gene Colan to create Night Force. Colan also worked with Doug Moench on Batman and Detective from 1982-84 or so. A lot of titles between 1980 and 1981 started to pay closer attention to continuity and long subplots in the storyline, but it was by no means universal at this point: Superman, Wonder Woman, Justice League, Superboy, DC Presents, and the like were still pretty much business as usual.

All-Star Squadron arrived, with Roy Thomas, in mid-1981, and continued to show how Marvel-type stories could be told in the Dc Universe. But you still had the anthology Dollar Comics like World's Finest, Superman Family, and the like, as well as the team-up books like Brave & Bold and DC Comics Presents, and IMFAO, so long as they had books like that, continuity was not going to be that important.

By mid-1982, DC seemed to be ramping up again with new series for Firestorm and Swamp Thing, and Flash was about to start on a long (two-years-plus) and compelling storyarc that was about the best that book had seen since Infantino left in the early sixties. But they were still doing stupid stuff like having Superman team up with The Masters Of The Universe in DC Comics Presents (issue 47).

In 1984, DC was coming along rapidly, converting Teen Titans, Legion, and Batman & The Outsiders to direct-only Baxter books, assigning Alan Moore to the Swamp Thing strip, adding new titles like Vigilante, Blue Devil, and Infinity Inc. Superman Family, Superboy, and Masters Of The Universe were long gone, and they were beginning to rework their old properties like Doctor Fate and Jonni Thunder into something more modern. And then, in April 1985, came the first issue of Crisis.

Shortly thereafter, basically every major DC character got a significant reboot, and continuity became more important across the line. World's Finest, Brave & Bold, and DC Comics Presents were all gone, as was the original Flash and Justice League. It was like a, ermm, New Universe....