The awe-inspiring opening cinematic, seeing the Lords use the power of their Great Souls to rise up against the Eternal Dragons who ruled the world. The storm of lightning spears sailing from Gwyn and his assembled army of knights is particularly thrilling, as is the sheer intimidating might of Nito, avatar of death.
There are enough cool boss battles in this game to have its own entry.
Anytime YOU as the player finally beat that one boss that took several tries to beat (Which is a lot).
Ripostes are immensely satisfying to pull off. The enemy strikes, but instead of simply blocking, you deflect the blow with your off hand. There's a thunk, time seems to slow as the foe staggers and is left wide open... and then you can run them through.
Sort of an off-screen example, but it's never stated just how long the Chosen Undead has been sitting in the Asylum before Oscar came along. It's very possible that they've been rotting away (literally) in some gods-forsaken cell, alone, for decades, maybe even centuries, and never went Hollow. The amount of willpower that would take is unimaginable. It puts your trials through Lordran in a new light, if the Chosen Undead has such an unbreakable will as to resist going hollow for god knows how long, then going through Lordran and pulling through all the crap it puts you is no challenge at all.
Hawkeye Gough, retired and blind, aids you by picking up his massive greatbow and sniping Black Dragon Kalameet out of the sky, grounding him so that you can fight him properly. The music helps sell it, but what makes it even more amazing is when he begins to feel around for his bow. You can literally see plumes of dust falling off of him and his bow. He's been stationary that long. And even out of practices as he might be he only fires once and hits Kalameet through the wing and square in the side.
This can turn into a Moment of Awesome if you work hard. Kalameet CAN be killed without Gough's help. It requires good reflexes, a strong bow, and a super fire resistant shield, but you can snipe Kalameet with arrows until he dies without Gough clipping his wings. If you successfully accomplish this insane task and then talk to Gough? When he would ask if you want his help with the dragon, he instead praises your skill for slaying the beast yourself, and expresses what an honor it was to meet you. Never has Developers' Foresight been so satisfying.
The Boss Battle against Sif! If you can catch it, Sif and Artorias are using the same moveset. Sif, a Canis Major, using the same moveset as the Master Swordsman Artorias. Let it sink in.
Siegmeyer, whom you were helping throughout half of the game, charges at the demons trapping both of you in the pit with his Zweihander and absolutely epic Battlecry, then casually takes a nap after they are defeated. Bonus points for being the only NPC who fights by your side not as a white phantom, but in his own flesh, meaning he risks his life to assist you.
The apple doesn't fall far from the tree. His daughter, Sieglinde, goes through hell and high water to track him down so she can make sure he dies in peace. She's not Undead.
After Knight Lautrec kills Anastacia of Astora and shuts down the Firelink Shrine bonfire, it is insanely satisfying to take the Black Eye Orb, track the bastard down in Anor Londo, and make him and his companions pay.
Any time you see the clever trap or ambush that was intended to take you out BEFORE it gets sprung on you, and turning that trap to your advantage, the first time you enter the area.
You can foil a great many ambushes if, instead of rushing towards that fleeing enemy, or that shining glimmer of an item, you stop, turn the camera about, and take a look at your surroundings. Turning the tables on an enemy ambush is always insanely satisfying.
Especially spotting the trap floors in Sen's Fortress before you trigger the arrows, and then lure the snakemen into the firing line. Done right, and you can make Sen's Fortress do all your work for you!
Against certain bosses, if you use a shield to block a powerful attack, if your stamina is high enough to take the blow, you are flung bodily, do a back flip, and land on your feet, with your shield still up. No matter how many times you see it, it is still insanely badass to watch.
Escaping the Gaping Dragon's grab attack. Let's explain: The Gaping Dragon's grab consists of the thing grabbing you, stuffing you into its immense, tooth-filled maw, and then chewing on you. And you can survive it. Probably with only a sliver of health, but still.
Moreover, by mashing buttons wildly, you can actually escape with minor damage only. Yes, you struggle out of that maw.
Beating the game becomes an accomplishment for the player in general, due to the sheer amount of stuff that you can find throughout your journey.
Of the two possible outcomes of your journey, picking the Dark Lord ending is insanely satisfying: you've been through hell and back, killing gods and overpowered beings all over the place, and at the end of it all,you give Gwyn's questionable plan a middle finger and become humanity's champion, bringing about an age where they can finally rule, free from those Jerkass Gods' dominion.
AotA made this outcome a Downer Ending for everyone save Primordial Serpents and possibly the protagonist.
Even long after the initial release of Dark Souls, new and impressively imaginative content is still being produced by its lively community. At the same time, most recent games start to reach the end of their lifespan, whereas Dark Souls appears to still be in the prime of its lifetime. Take Tanooki Suit's song "Lordvessel" for example, which is further upgraded with YouTube lore-ist VaatiVidya's epic montage (contains spoilers).
Most people mark Solaire linking the fire in his own world after helping you kick Gwyn's ass as a Tearjerker, but taken another way, Solaire doesn't just find his sun, he becomes the sun, and Solaire always wanted to be "grossly incandescent". You can't get any more incandescent than the sun!