Should Tidus master the Jecht Shot before arriving in Luca, the Aurochs ask him about it. He says, "It doesn't have a name. Anyone can learn it if they try," and the Aurochs immediately proceed to start practicing it.
Keep in mind that Jecht himself discouraged Tidus to even attempt to do his move, and said that he was the best. Tidus outright shot that notion down.
After being warned about the Chocobo Eater on Mi'ihen Highroad, Tidus suggests they go destroy it because it's "the right thing to do." Auron chuckles at the good memories it brings, stating that Jecht would always say the same thing, which meant trouble for him and Braska.
The story Kimahri tells about Yuna's first visit to the Moonflow, when the party is waiting to get on the shoopuf. It doubles as a Crowning Moment of Funny.
Kimahri: Shoopuf shook. Yuna fall in water. Shoopuf scoop up Yuna with long nose. Yuna jump in three more times for fun. ...Kimahri worry. Yuna: [apologetic, but giggling] Whoops. Kimahri: Yuna had fun. Kimahri happy.
Auron's story about Jecht getting drunk and attacking a Shoopuf becomes one when you remember the earlier scene where Young Tidus pleaded with Jecht to give up drinking. To elaborate: Jecht got drunk while Braska & co. were at the Moonflow, mistook a Shoopuf for a fiend and tried to kill it with his sword. Braska's party had to offer up all their money as an apology. After that, Jecht never touched another drop of alcohol again.
Seymour: What a pity. Ah, of course. "Protect the summoner even at the cost of one's life." The code of the guardian. How admirable. Well, if you're offering your lives, I will have to take them. Yuna:Maester Seymour. I trust my guardians with my life. But they are also my friends. I will not stand by and watch them be hurt. [pulls out her staff] I will fight you, too!
Cid and the other Al Bhed decide to blow up their Home to wipe out all the fiends inside. With great grief and sadness, they start singing the Hymn of the Fayth before destroying it. But Cid, his voice still cracking, states:
Cid:The great thing about machine—is that they can always be rebuilt!
A small one compared to others, but doubles as awesome: at Bevelle, Seymour attacks, and Kimahri holds him off so everybody else can escape. After running away for a bit, Yuna refuses to leave him behind, giving us this dialogue.
Yuna: I won't leave Kimahri behind! Auron: He's a guardian. Protecting you is everything. Yuna: Sir Auron! Tidus: That's right! We're all guardians! Yeah, and you know what that means? Yuna... Anywhere you go, I'll follow! Yuna: Anywhere I go? Tidus: Yeah, anywhere! Yuna: Well, then! Tidus & Yuna: Let's go!
Then everybody else follows and bands together to fight.
The cutscene "The Spring", in which Yuna and Tidus share their first kiss.
Let's just say it's not for nothing that the love story between Tidus and Yuna was, and still is, considered one of THE greatest love stories ever featured in a game. Yes, it's THAT touching and sweet.
Yuna, even after discovering the corruption and falsehoods of her religion, despite knowing that if she did so she would never be with the man she loves until his death continues with her pilgrimage. When asked by former Maester Kelk Ronso, before giving her passage through Mt. Gagazet, why she stills fights...
At Mt. Gagazet, the Ronso promise to bar Yevon's path and give Yuna as much time as she needs to reach Zanarkand. They then say they will erect a statue in her honour. One Ronso then adds "with great horn on head" and Yuna is so adorably pleased at this. It's also a nice little Mythology Gag since in the previous game in the series, summoners did have horns.
And the statue, complete with horn, is built in the sequel if you handle things with the Ronso and Guado right.
Before climbing Mt. Gagazet, the Ronso have one last thing to do to honor Yuna and her guardians: sing the Hymn of the Fayth.
Auron telling Tidus the truth about himself, and revealing why he continued to look after Tidus as a promise to Jecht. What really sold it was Jecht giving Auron a hug while calling him "a stiff", yet that's what he liked about him!
Yuna's Sphere that Tidus finds, which is meant to be her will before summoning the final aeon. In it she thanks Auron for telling Kimahri to stay with her, Wakka and Lulu for acting as sibling figures, and the "star player of the Zanarkand Abes", in whom she sensed something wonderful.
Special mention when she talks about meeting Kimahri as a child:
"Kimahri... Do you remember the first day we met? I was only seven. My father had defeated Sin, and all Bevelle was celebrating. Everyone was saying what a hero my father was. I was so happy. But when night came, it occurred to me. My father had defeated Sin and now he was dead. Now, I was all alone. I couldn't sleep, so I wandered into the town, away from the celebrating crowds. I stood on the bridge to Bevelle where my father and I had parted. Standing there, alone, I could see the fields where he had fought Sin. Then, you appeared, Kimahri. You said you were looking for 'the daughter of Braska', remember? At first, I was so scared. Until I realized what a gentle person you are. You weren't used to talking to children. When I told you that I was Braska's daughter... you said you would take me as far from Bevelle as you could. That it was the wish of a man facing death. I think... I cried then. Because that... that was when I knew my father was dead... and I would never see him again. You just held me, without saying a word. I cried after we got to Besaid, too. When you tried to go after leaving me in the care of the temple... I held onto you, crying, 'Don't go, don't go!' And you listened, Kimahri. You stayed. Kimahri, thank you. Thank you so much. And I've always liked your broken horn."
The above quote casts a bit more light on one of the game's more unspoken character interactions, that of Kimahri and Auron. While Yuna couldn't have known this, it becocmes apparent that the dying man Kimahri had promised to was Auron, not her father. After being mortally wounded in challenging Lady Yunalesca alone, Auron crawled all the way back to Mount Gagazet before he eventually died. Some of the last people he ever saw in life were Rin, and Kimahri. He asked Kimahri, a complete stranger, to watch over Yuna, presumably so that Auron as an unsent could fulfill his other promise to Jecht and watch over Tidus. Kimahri accepted, despite not knowing the man and Auron not having the celebrity that Braska had. Without Kimahri's gesture, Auron couldn't have fulfilled either promise in full. This puts their last interaction in a new light, as well. Kimahri knew the whole time that Auron was an unsent, but stayed silent because they had a mission and people to protect.
She doesn't mention Rikku because she recorded it near the Travel Agency of Mi'ihen Highroad, but she gives a great hug and thanks to her Al Bhed cousin at the end of Mount Gagazet.
After finally finding a solution to defeat Sin without the final aeon, Tidus throws the sphere out from the airship, as he knows she won't need to die for nothing.
Tidus:(holding up the sphere) You don't need this anymore, right? [Yuna is shocked from the revelation] Tidus: Right? (He throws it away. Both nod at each other.)
"My father... I loved him. So I... I will live with my sorrow, I will live my own life! I will defeat sorrow in his place. I will stand my ground and be strong. I don't know when it'll be, but someday, I will conquer it. And I'll do it without... false hope."
A couple from the airship near the end of the game:
First, when you talk to Wakka, he personally apologizes to Cid about his hatred of the Al Bhed. It's both heartwarming, and shows exactly how far he's come.
Second, before you leave to go fight Sin, Brother stops Tidus as he's leaving the room to say one last thing. "Rikku... you... guard." That's right, he learned a common language so he could ask someone to protect his sister.
Talking to Kimahri at certain points prompts him to say some very touching lines:
[Before going to Bevelle a second time]
Kimahri: Yuna, no apologies. Apology is running. Yuna never run away.
[Before fighting Sin near the end of the game]
Kimahri: You keep walking path forward. Kimahri watch the rear. Because of you, Yuna still smiles. Thank you.
[After entering the final dungeon and returning to the airship]
Kimahri: There is no need for fear. Kimahri will go with you.
After defeating Dark Valefor in the international/re-release editions, some of Besaid's townfolk will stand and yell at the summoner priest some of the same lines they said earlier in the story (the old woman's "Stay away from the summoner!" and the girl's "You're a bad man!") and he will run away cowardly. No matter what Yevon thinks of Yuna, she's still welcome in her hometown.
The only summoner who calls a Dark Aeon to emerge alive and unscathed is Dark Yojimbo's. After an epic marathon battle of 5 fights against Dark Yojimbo, the summoner admits defeat, prays for the party, and is allowed to go on his way.
Before Auron gets sent by Yuna, the part where he gives Kimahri, the character he's interacted with the least, a friendly nudge before going is really touching. The fact that his badass theme is playing helps.
It becomes more potent if you remember that, ten years ago, a dying Auron told Kimhari to take Yuna to Besaid, so he's pretty much thanking him for fulfilling his promise. Truly, much of their interaction remained unspoken. Kimahri was one of the last people to see Auron alive, knew he was an Unsent, and fulfilled the promise to Braska when Auron delegated it to him. The connection between the two was understood and mutually, maturely, respected.
At the end, after you defeat the Big Bad, Tidus tries to hug Yuna from behind. It's very sad and heartwarming at the same time because even though he is vanishing, he still cares enough for Yuna to try and comfort her before he disappears.
The 90+ hours of plot, Sphere Grid-grinding, monster hunting, chocobo racing, and blitzball are all justified by Tidus giving Jecht a high five in the Farplane.
The whole ending with Yuna's speech:
Yuna: Just... one more thing. The people and the friends that we have lost... or the dreams that have faded... never forget them.
An easily-missed example coming from an NPC: If you return to the Mi'ihen Highroad after acquiring the Airship, you can run into a Yevon monk who says he felt it was time to travel the lands and spread the teachings to those he meets. Talking to him again has him add that he plans to learn the Al Bhed language, in the hopes of opening a dialogue with them. After seeing the generally awful treatment most Yevonites - let alone the clergy - give the Al Bhed throughout the game, it's great seeing one of them being willing to extend an olive branch.
Lulu's comment to Tidus towards the end of the game, particularly after all the snarky comments she gave him initially.
Lulu: You've proven yourself as a guardian, maybe even as the best.
Lulu's relationship with Tidus becomes this over the course of the game; she slowly warms up to him, and ends up being an older sister figure of sorts to him. She starts naturally explaining things to him before he asks, and even expresses concern for him as early as Operation Mi'ihen.
Lulu: You were in contact with Sin again. The toxin - (gently) are you okay?
Yuna's comment to Dona about her numerous guardians and how much she treasures them. Particularly given how much she reveres her father.
Yuna: I only have as many guardians as there are people I can trust. I trust them all with my life! To have so many guardians is a joy, and an honor! Even more so than being my father's daughter.
Tidus' reaction to something he finally recognises in Spira: Blitzball! He's just been pelted in the head by Wakka's wayward blitzball, and he yet he has the brightest smile on his face as he exclaims "Blitzball!"
Finding Jecht's Sphere in Macalania Woods. It shows snippets of Auron, Braska and Jecht's journey across Spira. Auron tells Tidus that Jecht wanted to take lots of pictures, to show him when he got home to Zanarkand. The Sphere ends with Jecht addressing Tidus.
Jecht: Hey. If you’re sitting there, watching this... it means you’re stuck in Spira, like me. You might not know when you’ll get back home, but you better not be crying! Although, I guess I’d understand. But you know what? There’s a time when you have to stop crying and move on. You’ll be fine. Remember, you’re my son. And… Well, uh… Never mind. I’m no good at these things.
Afterwards, Auron stops Tidus to tell him that Jecht really did love him. Tidus brushes it off, but does add a "thanks" to Auron's retreating back.
Rikku admires Lulu's maturity, and says that she hopes to be like her when she grows up. Kimahri simply states that "Rikku should stay Rikku." Rikku initially takes it the wrong way, but it's sweet that Kimahri has already grown attached to Rikku during their short time together.
When the party sans Yuna is stranded in the Bikanel desert, Tidus starts out alone. However, Auron soon joins Tidus mid-battle to help him out, and Lulu follows shortly afterwards, asking if Tidus is okay.
A minor case with an NPC character from Besaid: there's an older man who works in hut creating and weaving fabrics, and is extremely hard working, it seems that he never leaves the loom to stop doing his job regardless of what's going in on the island or in the world, even when Sin is battling the airship and the prospect of an Eternal Calm is looming. However, it is stated that he did leave the job once during a workday: he left to see Yuna off when she first left Besaid to start her pilgrimage.
The laughter scene starts out funny but becomes sweet once you see Yuna join in with Tidus's ridiculously fake laughing.
A surrogate brother and sister relationship develops between Rikku and Tidus throughout the game, and by Mt Gagazet it's clear that they are incredibly close. If you ride with Rikku to Macalania Temple, she realises he doesn't know the truth about the Final Aeon and gives him a little hug. Tidus is surprised, but finds it nice nonetheless.
The Tumblr "How Games Saved My Life" has two separate stories about how the game helped fans get over trauma: One identified with Tidus, helping him get over his own absent father, while another was inspired by Yuna to escape an abusive relationship.
This Escapist article, where the writer brings several games to a homeless shelter, including Final Fantasy X. Several homeless Polish men enjoy simply running around and enjoying the game's scenery, tearing up because Besaid looks so much like home.