That your Bard will whistle, when they have no instrument equipped.
The sound Scratch's ball makes, when you move it in the inventory.
The whispering of Soul Coins.
The Bard's solution to the interrupted couple in the barn and the door at Waukeen's Rest.
Wild Magic Sorcery - especially when it turns everyone into Cats & Dogs.
The random items, which might look like grey merchant-loot on first glance, but can be combined into weapons or utility items without any associated quest telling you to do so.
That you can find out what happened at Reithwin all those years ago without any quest-prompt to puzzle the pieces together.
The dialogue encounter of Thisobald Thorm. Sends chills down my spine each time.
The Brain Jars.
Stopping the Press - such a good stealth dungeon.
"Like moons make swell and wane the nescient seas, so too the sky-strewn gods ordain the tidal fates of mortal days. And yet - a notion born in lonely hours - come ebb, come flow, come all that is beyond the breadth of our dominion: be a moon unto yourself."
Basically all of Gale and Lae'zel's writing.
A lot of Gale things:
That he has a Tressym-corner in front of his tent, should Tara drop by and that he included her pet bowls in his illusion of Waterdeep.
The "In Case of Death" quest
The whole Thrills of Battle conversation. It came so completely out of nowhere the first time and now is a moment I look forward to.
Having a self destruct cantrip I can use instead of F8 when things really haven't gone as planned.
The flirty banter in most of his romance scenes, especially the drunken conversation at the party and the Waterdeep section. The player dialogue is very nicely written in these scenes too, which gives them the flow of an actual conversation. I love the playful warmth of these interactions.
That in his Origin
you are given an awesome ring right at the start and then are low-key forced to destroy it or limp around with a penalty for a while because you rarely have any other items to snack on at this point in the game. It made his condition feel like a problem for a change.