Secret appearing in an episode appropriately written by Peter David. She's the first such Young Justice comics member to appear in the show — if one doesn't count Superboy, since his cartoon characterization is drawn from his later Teen Titans days and much unlike his earlier self.
Fans of Teen Titans were excited when they found out Garfield Logan a.k.a. Beast Boy was going to appear.
Superman warming up to Superboy in the end of Season 1.
There was much rejoicing when it was revealed that Jaime Reyes would join in Season 2.
Tim Drake coming in as the new Robin while Dick grew up to become Nightwing.
Wonder Girl (Cassandra Sandsmark) finally being used in a DC animated show. Like Tim Drake and Superboy — and unlike most of the cartoon team — she was in the original Young Justice comics, all the more reason for some fans to rejoice. (At the same time the original Wonder Girl, Donna Troy, is in Super Best Friends Forever).
Whenever a season 1 team member that wasn't immediately seen in the season 2 premiere appeared later.
Lian Harper exists.
Bart Allen/Impulse has arrived, again in an episode appropriately written by Peter David.
In the same episode that Wally returned as Kid Flash, and Jay Garrick appeared as the original Flash, in a team-up with all four of the Flashes. Needless to say, with DC's treatment of Wally followingBarry's return inFinal Crisis and the events of Flashpoint seemingly erasing Wally's tenure as a superhero from existence, the episode in general was very well received by fans of the Flash family.
And as of the SDCC 2012 trailer, we've got Static and Deathstroke, Artemis in a Tigress costume, and possibly Black Beetle, who has been reworked to be a part of the Reach.
Fans rejoiced and wailed at the same time with the revelation that Jason Todd exists...and already died.
Arsenal makes his first appearance on tv!
Stephanie Brown also makes her TV debut.
The one time the fandom did not rejoice? When Cartoon Network preempted Before the Dawn and then cheerfully informed fans that the show would not air again until January of 2013.
That said, the fandom did rejoice when, due to the hastiness of their decision and the fact that they didn't tell anyone else, iTunes still put the episode up as scheduled, and it peaked at #2 on the sales charts and was allowed to spread over the web. They pulled it, but it's now accessible for anyone who wants to see it.
"Intervention" was a tribute to the legacy of the Blue Beetle, mentioning both Dan Garett and Ted Kord. In addition, the Scarab showing his respect for Jaime once taken off Reach control. Khaji Da, anyone?