In no particular order:
Grant Morrison's return to the character that helped put him on the map, Animal Man.
52 re-instated Animal Man to the DC Universe.
Adam Strange, Starfire, Lobo and Animal Man as an intergalactic Fantastic Four.
It introduced Batwoman.
Good or bad, DC Comics got actual media coverage.
DC's gracious handling of Batwoman's media coverage.
It had a mysterious island full of evil scientists including Dr. Sivana.
It had Veronica Cale, world's greatest Wonder Woman villain, in it.
Black Adam's wordless dismissal of Veronica Cale on Oolong Island said more about who he is than any thousand words could.
They were going to kill Veronica Cale in 52 but at the last minute, they chose not to do so.
"Who is Supernova" was one of the best comics detective stories ever to be published outside a Batman comic.
Ralph Dibny's acting skills.
Grant Morrison writing Lobo.
Greg Rucka writing The Question.
Renee Montoya was set-up to take up the mantle of The Question.
The back-up origin stories featuring artists closely associated with any given character, example: Adam Hughes illustrating Wonder Woman and Power Girl.
The rise and fall of The Black Adam Family.
Pound-for-pound one of the creepiest moments in comic book history: Wicker Sue Dibny.
Ralph Dibny outsmarting Neron, something it took the combined DCU to do ten years, prior.
T.O. Morrow's reminding us exactly who he is:
The set-up of Booster freaking Gold's new role as "the greatest hero you've never heard" of within The DC Universe.
The sheer brilliance of Lex Luthor's "Everyman Project."
The utter horror as he deactivates the project over the Metropolis skies.
DC Comics has a Crime Bible. A Crime Bible.
The return of Skeets.
The Great Ten.
Black Adam and American soldier/intergalactic cop Hal Jordan discussing global diplomacy.
Black Adam's use of Terra-Man as an example to all.
The idea that The Persuader is a villain legacy that will last clean into the 31st Century.
Kandor plays a fairly prominent role.
The "nostalgia" of an Isis within a DC Comic.
Ralph and Sue Diby, Ghost Detectives.
Black Adam declares World War III.
The cementing of Black Adam as one of The DC Universe's greatest of super-villains.
The cementing of Black Adam as not truly being a super-villain but a power unto himself.
Booster Gold going back to his old football days and calling his final football play, "52."
The Blimp, Oddman and Beefeater as pallbearers at Booster Gold's "funeral."
The re-establishment of The Monster Society of Evil.
The shock of the death of Osiris.
Proof-positive, once and for all, that you can't just name yourself The Justice League.
Even if it was only for two pages, someone made Halo from The Outsiders a pretty interesting character.
The idea that The DC Universe is bigger than Superman, Batman & Wonder Woman.
It gave us the return of The Multiverse.
The stories told in cover artist J.G. Jones' 52 covers.
52 consecutive issues in 52 consecutive weeks.
DC Comics was seen as being innovative for the first time in decades.
Solid art from people who understood the importance of getting the art in on time.
Keith Giffen's role as storyboard artist gave each issue, no matter who the artist, continuity.
The anticipation of every new issue.
We will never see something of this scope again wherein four of comics' most talented writers, Mark Waid, Grant Morrison, Greg Rucka and Geoff Johns, work together and at the top of their game to bring us one story, over one period of extended time.
We simply didn't know how good we had it.