At the funeral of Deckard Cain, Leah bitterly brushes off the attempts of Tyrael at comforting her, rebuking "What would you know about sacrifice?" He took her hand and showed her through his eyes how he was willing to defy Imperius, and hence Heaven Itself, to give up his status as an angel to protect the humanity he so loved. Leah was so moved by this gesture of love and kindness that she shed Tender Tears at the revelation.
This exchange between Lyndon the scoundrel and the barbarian:
Scoundrel: "If I keep helping you kill the lords of Hell maybe I'll be a good person, eh?"
Scoundrel: "Are you serious?"
Barbarian: "You already are one."
Another exchange involving the scoundrel, with the Witch Doctor:
Scoundrel: "What would you do without me?"
Witch Doctor: "Miss your company!"
Counts as a Call Back since there is an Act 1 exchange in which Lyndon muses what the Player would do without him. The response being "Enjoy the quiet."
This little exchange shows that in spite of finding the Monk to be stuffy and far too serious, Lyndon the Scoundrel really cares for the Monk as a friend:
Lyndon the Scoundrel: You look so grim, isn't this your noble quest?
Monk: I do what I must, why?
Lyndon the Scoundrel: Well... you worry me at times.
In Act III there is a an older soldier and his wife who have several heartwarming conversations. In one she congratulates him for throwing the demons back.
Soldier: "You know, there were other soldiers out there besides me."
Wife: "I didn't notice."
Made all the more heartwarming by their previous interactions as an old married couple, with all the bickering and stinging that that implies. Then Diablo goes and kills the both of them, and you find the wife's burial wishes, where she very calmly states that they're both dead, and want to be buried together, but without the soldier's sword, because "he won't be needing it anymore".
Random dialogue with the Templar as a Demon Hunter elicits a mention.
Templar: "Your valor places you amongst the highest ranks in my order."
Demon Hunter: "Thank you. But I am more proud to call you my friend."
Templar: "As am I."
Gets even better when you remember back in Act I, when the Demon Hunter really did not like the idea of having company.
The following conversation with Lyndon the Scoundrel near the end of the game, which is a major Pet the Dog moment for the player character:
Player: I have a gift for you, Lyndon.
Lyndon: Is that so?
Player: Take this gold. When this is all over I will go with you to Kingsport. There you can pay your debts and your brother will be free.
Lyndon:*Genuinely touched* You are much too kind.
Player: We have journeyed together a long way. I am happy to help you.
Lyndon: Words fail me.
Player: Words are not always needed.
Kormac the Templar during the final stages of the game is a much humbler man than he is when he first joins the player character, openly admitting that he misjudged both Lyndon and the PC and apologizing to both for his arrogance. Aww.
Mayor Holus' little Character Arc: When we first meet Mayor Holus of New Tristram, he is stuck in the middle of town, because his cart broke and blocked the street as he was trying to flee. The Nephalem has little respect for a coward who would abandon his town in its hour of need. Then we meet him again at Bastion's Keep, and he has not changed a bit, still thinking to flee the fortress before the fighting begins. The Nephalem, fed up, gives him a piece of their mind, to which Holus replies that he simply wants to live. Later, as the demon attacks wane, Holus has his chance to run, but wonders if it wouldn't be better to fight for his life instead of running forever. Surprisingly, he keeps to his word and stick around, eventually dying in the final struggle at Bastion's Keep. A nearby guard says that he never did run, even though he had the chance to at one point.
There is a mini plot arc in Act II of an exiled noblewoman and an Iron Wolf swordsman. She starts off as a Rich Bitch with bits of Fallen Princess, while he is a Deadpan Snarker to her. He helps her survive her exile despite his distaste for nobles and she becomes a Defrosting Ice Queen. He saves her during the Harrowing of Caldeum and the two become an official couple.
Your final conversations with Leah in the Armory as she is trying her hardest to keep the Black Soulstone together with all Hell trying to get in. Turns into a major Tear Jerker later though when Adria's betrayal goes down.
Freeing Auriel from Rakanoth in Act IV not only restores hope to the angels of the High Heavens, but also to the people of Bastion's Keep, who had been hit with major despair following Adria's betrayal and Leah's death.
Going through Act IV with Lyndon, your character points out his gold gathering has become rather lax. He basically replies "Screw the gold, this is for Leah." For all his debauchery and shameless come ons, he genuinely cares about her.
To the point that if he ever has a daughter, he's naming her after her.
One Lyndon's Closing Dialogue after you defeat Diablo: