By Phase 2 of the Marvel Cinematic Universe, post-credit scenes had become a staple of the franchise. Marvel also made the decision to include scenes in the middle of the credits as well, giving fans two extra scenes per movie instead of one.
Rather than building up to one project like how all the post-credit scenes of Phase 1 led to The Avengers, the post-credit and mid-credit scenes of Phase 2 were used to tease various storylines and characters, most of which wouldn't be seen until Phase 3.
Thor: The Dark World (Jane and Thor reunite)
Jane and Thor have had a complicated relationship due to him struggling to balance his responsibilities on Asgard and his desire to be with her. In Thor: The Dark World's post-credit scene, Jane is sitting in her apartment, visibly sad after Thor left again, before hearing thunder crack and looking out onto her terrace to see the Bifrost. She rushes out to greet Thor and the two share a kiss before it cuts to the frost beast, that was transported to Earth during Thor's fight with Malekith, tripping over a shipping container while chasing a flock of birds. No one knew at the time but this was the last time fans would see them together due to their off-screen breakup that was revealed in Thor: Ragnarok.
Guardians of the Galaxy (Howard the Duck)
In the post-credit scene of Guardians of the Galaxy, The Collector sits in the ruins of his museum which was destroyed when his assistant, Carina, attempted to grab the power stone, killing herself in the process. As he drinks from a glass, a now free Cosmo the Space Dog comes over and begins licking his face when Howard the Duck, who was previously in a glass case of his own, asks why he let him lick him like that, calling it "gross" before taking a sip of his own drink. This scene didn't really have much of a purpose beyond being an easter egg for Howard the Duck, a character that not many people were aware existed in the MCU before then.
Iron Man 3 (Bruce Banner as a therapist)
Iron Man 3 is told from the perspective of Tony Stark and even has him narrate the beginning and end of the movie. However, it's revealed that he was actually recounting the events to Bruce Banner when Bruce jolts awake and admits that he fell asleep near the start despite Tony complimenting him seconds earlier for being a good listener.
He reminds Tony that he isn't "that kind of doctor" leading Stark to continue rambling about another topic to his annoyance. While the scene is mostly used for comedy, it was a great "Science Bros" moment that would not have fit in any Avengers movie.
Ant-Man (Civil War teaser)
Much like the post-credit scene of Captain America: The First Avenger, the post-credit scene of Ant-Man was an edited clip from an upcoming project, Captain America: Civil War. It shows Sam and Steve talking about what to do with Bucky. While they both agree that Tony he might not even believe them and that even if he did the Accords might not even let him help, Sam says that he "knows a guy" that could help them, referring to Scott Lang aka Ant-Man, who Sam lost a fight to at the Avengers compound and who he later reached out to through Luis. Not only was this the first reference to the Sokovia Accords, but it was also the first hint that Ant-Man would be returning sooner than expected.
Captain America: The Winter Soldier (Bucky at the museum)
The biggest reveal from Captain America: The Winter Soldier was the fact that Bucky Barnes didn't actually die in 1945 and had instead been used as a brainwashed HYDRA pawn for decades. While it seemed that the character fans met in Captain America: The First Avenger was gone, the post-credit scene provided a bit of hope that the real Bucky was still in there somewhere. After going on the run, he secretly visits the Smithsonian Institution to look at an exhibit dedicated to the man he used to be, similar to how Steve did earlier in the movie. This scene marked the beginning of Bucky's journey to rediscover himself and try to get back to the person he was before, one he is still currently on.
Ant-Man (The Wasp suit)
In Ant-Man, Hope spends the majority of the runtime trying to convince her father, Hank Pym, to let her go into the field. While he initially refuses in order to protect her, in the end, he takes her to a secret vault and reveals a prototype of the Wasp suit that he started with her mother, Janet. He tells Hope that he believes that were working on it for her and that they should finish it together, setting up her debut as Wasp in the movie's sequel. While the suit ended up looking completely different when she did show up, it was great to know she would eventually get to assume her hero identity from the comics.
Thor: The Dark World (The Collector's introduction)
After Thor: The Dark World, Sif and Volstagg travel to deliver the Aether to Taneleer Tivan aka the Collector on Knowhere. Upon arriving at the Collector's museum, he bows to them before asking why it wasn't kept on Asgard with Volstagg replying that it isn't wise to keep two Infinity Stones so close, referencing the Tesseract.
After commending the decision, Sif hands him the Aether and he promises that it would be safe with him, saying "one down, five to go" after the pair leaves. While the scene officially confirmed that the Aether was one of the six Infinity Stones, it also the Collector's first appearance before going on to show up in Guardians of the Galaxy and Avengers: Infinity War.
Guardians of the Galaxy (Baby Groot dances)
Groot's death to save his friends was one of the saddest parts of Guardians of the Galaxy but, luckily, one of the twigs from his body was able to be saved by Rocket and was put in a pot so that it could grow. In the mid-credit scene, a newly sprouted baby Groot is aboard the rebuilt Milano, dancing to the Jackson 5's "I Want You Back" while Drax sharpens his knives, hilariously freezing in place when Drax looks at him to catch him moving and then going back to dancing when he looks away. While it doesn't reveal anything new or exciting, it still remains one of the funniest scenes from the franchise.
Captain America: The Winter Soldier (Quicksilver and Scarlet Witch are introduced)
Captain America: The Winter Soldier saw the disbanding of SHIELD after the discovery that the organization had been operating as a front for HYDRA for decades. At one of HYDRA's hidden research bases in Sokovia, two of the remaining members of the organization, Doctor List and Wolfgang von Strucker, discuss SHIELD's fall and the possibility of them being discovered. It's then revealed that they were in possession of Loki's specter and were using it to run experiments on human volunteers, only two of which, Pietro and Wanda Maximoff, had survived. Both are seen testing out their new abilities with Pietro using superspeed to run around his cell while Wanda levitates some blocks before smashing two together. In addition to showing that, while Cap was able to dismantle SHIELD, HYDRA was still out there and wouldn't be completely snuffed out until this facility was attacked and the specter was recovered in Avengers: Age of Ultron, it also gave fans the first real look at Quicksilver and the Scarlet Witch.
Avengers: Age of Ultron (Thanos picks up the Infinity Gauntlet)
The post-credit scene of Avengers: Age of Ultron showed Thanos walk into a vault, put on the Infinity Gauntlet, and utter the now-iconic line, "Fine, I'll do it myself" with a grin on his face as he clenched his fist. Beyond showing that Thanos was still going to be a major character in the future of the MCU following his appearance in Guardians of the Galaxy, it also showed that the mad titan was going to make more of an effort to seek out and find the Infinity stones without relying on characters like Gamora and Ronan to do it for him.