The Giant Blacksmith will consider the player his friend should they talk to him often.
"You good friend. I very happy."
When you first meet Rhea, she speaks very coldly at you, even calling you uncouth. After you rescue her from the Tomb of the Giants, it's really heartwarming to see how much she has warmed up to you. It even seems like you're the only one she trusts by now.
Quelana's gratefulness towards you before she departs, specially this line:
"Our time together was short, but sweet..."
If the Chosen Undead is not a pyromancer, Laurentius of the Great Swamp—who is in the running for the sweetest character in the game—will give them a piece of his pyromancy flame. This seems like a thoughtful gesture at first, but its magnitude isn't immediately apparent unless you speak to him again: a pyromancer's flame is born from his essence. When he shared that piece with the Chosen Undead, he literally gave them a part of himself. The tone of voice he uses to explain this really sells it. Amongst other things.
Any time two or more players engage in jolly co-operation to beat a particularly tough boss. Even more so for the one doing the helping, especially if the one they helped gives them a bow or a Praise-to-the-Sun before the phantom departs.
There can even be cases of this with invading players. For example, being invaded at the Undead Burg and the invader dropping Titanite Shards, which aren't normally available that early.
Dusk of Oolacile's gratefulness to you after you rescue her from the Crystal Golem. And her happiness at being summoned by you after being alone for so long.
"My home of Oolacile was reduced to ashes, long ago in my time. I have been alone ever since. But to be summoned thus, and be of service to thee...it is...most rewarding."
It gets better if you complete the "Artorias of the Abyss" DLC. When you see the state of her time period, it becomes evident that besides Elizabeth, you are her only friend, and a little thing like a few centuries of time apart won't keep you from reaching out to her. And you rescued her again for good measure. In a game as bleak and filled with Black and Grey Morality as Dark Souls, a trope as old-fashioned and cliche as Save the Princess comes across as almost bizarrely satisfying.
If you donate enough Humanity to the Daughter of Chaos, eventually her voice will lose the in-pain tone. Even if its only a small amount, you've managed to alleviate at least some of the poor girl's suffering.
The Daughter of Chaos really seems to act as a Morality Pet to much of the community. Case in point, some truly dedicated players have hit the upper cap on how much humanity you can donate to her. That cap is 60,000. Just to put things in context, there is no gameplay bonus for helping her beyond 30 humanity, and the cosmetic bonuses stop at 80 humanity. So players gave her tens of thousands of humanity simply because they wanted to help lessen her suffering.
And after they hollowed, they absolutely refused to attack her, if you take the fog wall as being there just for gameplay reasons.
Learning the truth about Artorias' sacrifice. He never made any deal with the Abyss, ventured into it to save Oolacile from it, and abandoned his shield to protect Sif from its Dark. Then Manus broke him. He was always on humanity's side, even if Gwyn wasn't. In the cut dialogue for Artorias, he would have praised the chosen Undead's skill and beg you to save humanity from the Abyss before it spreads. Even while suffering from the possession, he still has his compassion and willpower.
If you rescue Sif as a cub in the past then face him in present, Sif will actually get the drop on you. But he refrains from finishing you off when he recognizes you. Sadly, he can't let you go without a fight without violating his oath, but he at least gives you the chance to defend yourself.
Any time you're invaded or invade someone else, and both combatants give a bow before fighting, often times waiting for the other to finish completely. It's a very simple gesture, but one that can't be interrupted and leaves you open for attack should the other person be feeling a bit cruel. In short, a potentially very stupid move. Regardless, the majority of players do it purely because it's the most sporting option available, and a general indicator that what will follow will be a fair fight.
While a minor example, seeing a message written by another player that simply says "I did it!" after an area where a particularly tough foe or a boss was located can count.
Assisting a player with either the Bell Gargoyle or Quelaag boss fight will usually send you back just in time to hear the bell ring in your own world. That's thanks to you, and you should be proud.
Likewise, hearing the Bell of Awakening tolling in Undead Parish or Blightown can bring a small wave of pride, knowing that someone - possibly a new player - has overcome a major milestone with or without your help.
Oscar of Astora's final words. They don't lament, they don't grieve, they don't complain or regret. At most, he's just thankful that you can make it out where he failed.
"I would hate to harm you after death. So, go now...and thank you."