For one, all of the other residents are nothing but welcoming to Bloo (unaware of what a nuisance he'll eventually become). Wilt willingly forfeits his bed to him and Coco lays him an egg with a photo of Mac inside when she sees him having trouble sleeping.
Mac keeping his promise to come back and visit Bloo at the house, proving Mr. Herriman wrong and prompting Frankie to continue trying to melt his frozen heart.
Mr. Herriman: It's just a cruel fact of life, Miss Franciss. Every child tires of their imaginary friend eventually.
Frankie's status as Mac's Cool Big Sis. As has been pointed out elsewhere, Mac's life is one that has driven him incapable of imagining security that doesn't involve some kind of abuse. And yet here is this woman who has no reason to be anything more than polite to him but chose to be the closest thing Mac has to a loving older sibling and even in some ways a Parental Substitute.
After the climax of "Destination Imagination", Frankie wants to free the antagonist, a Reality Warper with serious separation issues who'd destroy any would-be rescuers so long as Frankie stayed by his side. Mac, Bloo and the gang, who have just narrowly escaped his giant monster form, try to talk her out of it. Herriman, who has been absolutely brutal to Frankie (even moreso in this movie than throughout the rest of the series), tells her, "Miss Frances, I have known you since you were a little girl. As I have watched you grow, I've been amazed at your lack of maturity, laziness, and lack of judgement. And all that time...I was wrong." He goes on to say that if Frankie wants to set him free, he trusts her judgement. Later, when the villain-turned-good-kid tells Herriman Frankie is a wonderful person, Herriman lays his hand on her shoulder and agrees, "That she is." It's not as touching if you don't know the characters as well, but to any long-time fan of the series...Ocular Gushersto the max.
Not to mention Frankie sewing a body just for World so he doesn't have to jump around at everything all the time, and that the toy chest sort of becomes a playground for everyone at Foster's - with Herriman and Frankie shown to gleefully jump in too. It's wonderful.
Even better, if you take "Destination Imagination" as the finale, it shows ALL of the friends running through the hall like they do in the intro and ending with one big party. And having it in the toy chest says to all viewers "They're right here whenever you want to play."
The ending of the episode Go Goo Go qualifies too. DI's just still happens to top it. But in case you're wondering how the ending of the former goes, Mac comes into the room where Goo is crying, asking if Goo is okay. It turns out that Goo felt lonely and that's why she created all the IFs. Mac gave her a speech about how imaginary friends are real friends and if she got to know the ones she already had then it would likely alleviate her feelings of loneliness. Mac, not realizing that the speech had cheered up Goo enough that she's already smiling, then apologizes for his Minor Insult Meltdown (the cause of her crying in the first place) and tells Goo that he wants to be friends with her—which causes an excited Goo to hug Mac. In the aftermath, Goo is finding homes for the IFs and has become the friend of the month as a result, while Mac has joined in on her wackiness.
Herriman's 'Funny Bunny' routine, though funny and at times cringeworthily saccharine, is made really heartwarming by the fact that this is Mr. Herriman we're talking about. He's still Madame Foster's imaginary friend even after all those years. Awww.
And even after all those years, he still sees her as that "sweet little girl." Double aww.
After spending most of the episode considering the performance an Old Shame and failing to prevent the footage of it from leaking onto the Internet, it turns out that the online viewers enjoyed it the same way Madame Foster enjoyed it (as opposed to laughing at him), so he starts performing it for the child customers. Triple awww.
At the end of the episode when Harriman reveals his new hip-hop version of it, everyone walks out of the room in disgust - except for Madame Foster, who's enjoying it as much as Herriman.
The ending of "Bye Bye Nerdy".
Good Wilt Hunting TO THE MAX.
The Sweet Stench of Sucess, Bloo becomes a celebrity, but can't see his creator, Mac, which leaves Mac heartbroken. By the end of the episode, they meet up again, and even a Jerkass like Bloo admits that he loves Mac more than anything.
No mention of "Who Let The Dogs In?" That moment near the end, before the happy twist, where Frankie gently takes the last puppy from Eduardo, sadly saying just "I'm sorry, Ed..."
The book end of the series. The opening animation shows the house being drawn, and the closing animation of the episode "Good-bye To Bloo" shows the house being un-drawn, as if to say "Thanks for watching, but all good things must come to an end." Followed by a thanks from Craig McCracken and Lauren Faust in the closing credits.
In "The Big Picture," Camery laments about not being able to be a part of the yearly Foster's Home photo because he's the one taking the photo. At the end of the episode, Herriman has a mirror-friend stand directly across from him, so he is in the photo.
In "Busted," Bloo was under the threat of expulsion from the house from Herriman if he kept breaking the rules. After accidentally breaking Madam Foster's bust, Wilt offers to take the fall for Bloo, which he politely refuses. Considering that this was after Bloo's Flanderization started, you'd expect him to jump at Wilt's offer.
At the end of the episode, when Bloo is found out, Mr. Herriman gets a little Pet the Dog moment that reminds the audience that he's still a Reasonable Authority Figure at his core. At first, it seems he's going to discipline Bloo for breaking the bust, but it turns out that imaginary friends break it by accident all the time and that, since he had hundreds of duplicates made for speedy replacement, he assures Bloo that he had nothing to worry about.
Although not heartwarming in the usual sense, the times throughout the series where Herriman and Frankie got along and worked together.
In the pilot, after Bloo goes on a melodramatic (but still sincere) plea explaining him and Mac's situation to Herriman, the old fuzzball is silent for a moment, but relents and agrees to let Bloo in, clearly moved by their plight.
Meta Example: In real life Tom Kane is an active adoption advocate, he and his wife have taken in many foster children. So when Mr. Herriman talks about finding new homes for imaginary friends, you know its coming from a sincere place.
Seeing Mac, Goo, Bloo, Wilt, Coco, and Eduardo playing Pretend in "Make Believe It Or Not" will make you smile. It's just cute and funny to see the friends enthusiastically killing time on a rainy day with a childish game that goes over-the-top. The kids are being kids and the imaginary friends are being the playmates and best friends they were made to be. Brings one back to the days of childhood innocence.
In a rare villainous example, there's a brief moment in "Seeing Red" when Terrence nearly cries Tears of Joy as the titular Red goes to "SMASH BLOO!!!"
In the same episode, Bloo actually apologizing to Red after he put him through so much misery for the entirety of their tour around Foster's after realizing that Red was much, much different and was considerably nicer and softer than his creator.
How well Mac and Bloo were getting along in "The Bloo Superdude and the Magic Potato of Power!" Mac was totally invested in that story!
In the episode where they were taking a group photo of all the friends, it's demonstrated early on that Mr. Herriman is insistent on having everyone in alphabetical order, as shown when he denies Wilt's request to stand lower on the stairs so he can be seen. However, at the very end, he makes a single exception: he tells a mirror imaginary friend to stand lower so the camera imaginary friend, who's taking the picture, can be in it as he's always wanted.
When Uncle Pockets is discovered to be keeping mementos of his past adopters, he thinks this will end everyone's respect for him. The rest of Foster's is quick to say otherwise.
Mr. Herriman: That's preposterous! There is no shame in loving your child!
Madame Foster:(walks up to the pile and picks up a dolly) Collecting mementos is a wonderful endeavor!
Other Imaginary Friends: And to us you'll always be/the best imaginary friend ever!
Mr. Herriman's comment on the matter is especially sweet, considering his relationship with his creator...
After Bloo has his Heel Realization in "Cuckoo for Coco Cards," he goes to apologize to Coco, and makes sure Coco knows he means it with this:
(Having pulled out his stack of Coco Cards) Coco? I love these cards. I love them more then life itself! But if they mean we can't be friends, then... (Throws all the cards into the fireplace)
The "Let Your Hare Down" episode is an example of this, most particularly for Bloo. Even though that Frankie with Mac and the others had schemed with Bloo to do this, it's the simple fact that Bloo goes through many different trials to try and make the normally uptight Mr. Herriman lighten up. Keep in mind, this is Bloo we are talking about; someone who's so selfish, he'd sell his friends (except maybe Mac) out if it meant satisfying his own desires... and he goes through every possible fun thing he can think of just to see Mr. Herriman smile, ergo, he's doing something for not only his own benefit, but for Herriman's and those at Foster's who believed he needed to stop being so uptight.
Of course, Bloo does manage to succeed in getting Mr. Herriman to loosen up by unintentionally having the chickens attack him, however... it seems to work a little too well as Mr. Herriman takes loosing up way too far, to the point that things like bills don't even matter to Herriman anymore.
Harriman learning to respect Coco and not treat her as a nuisance.
Despite the episode itself being controversial, Foster's House for Make-Em-Up Pals did have a sweet moment at the end—when Goofball turns out to actually be an imaginary friend and Frankie has to admit she was wrong, Goofball gives her a sincere thank you for taking care of him while he was there, something she's genuinely touched by. And given how much emphasis is placed on how thankless Frankie's job is, this must have meant a lot to her.
Heck, the series' entire opening sequence qualifies! After Foster's is drawn into existence, Mac walks up the house's stairs and meets up with Wilt, Coco, and Eduardo, who are all pleased to see him. We then cut to Bloo, who's just standing there bored, but is visibly happy to see Mac and we see all the friends laughing with joy.