Accidental Nightmare Fuel: In the Section 2 Obliteration boss fight against Ness and Paula, neither human has a Check description, only stats, giving off the impression that Kris is at a loss of words over the gravity of the situation. In actuality, Word of God reveals that Sarah simply couldn't think of a description that fit and forgot to add one before releasing the section.
The game has a well made custom battle interface which is basically Undertale's interface modified to accommodate multiple party members and the TP system.
The Undertale styled custom sprites made for the later sections are well designed as well, especially the battle sprites and dialogue portraits.
The sprites (and in Section 7's case, models) made for the Delta Warriors in the styles of other games are nearly flawlessly done in their styles. Special mentions to Section 6's Bowser's Inside Story styled sprites for Kris and Susie, all made possible thanks to Cyberstar.
Rude Encounter is an incredibly well-done mashup of Enemy Approaching and Rude Buster (which is retooled to the more chiptune-esque instrumentation of the former).
FLOWEY is an amazing boss battle theme that incorporates, among other things, the Yellow SOUL portion of Your Best Nightmare, Your Best Friend, Heartache, ASGORE, and even a minor-key version of Once Upon a Time. The whole piece perfectly encapsulates what battling a SOUL-less but still powerful base form Flowey must feel like. After this fight, the player should expect more epic bosses in the future.
Battle Against A Rude Opponent is basically Rude Buster if it was played on an SNES and it blends with the Earthbound music very well.
Ruder Monsters Takes the original Hard Mode theme, Stronger Monsters, and adds more intensity with subtle bits of Rude Buster in the background as well as replacing certain segments of the song.
The Gauntlet Of Falling Terror is a frantic theme that plays as the divergence caused by Sans's attempt at attacking Kris at the bridge resulted in Noelle falling off, forcing everyone to jump after her as you have to dodge the Gauntlet weapons thrown at you.
Eye For An Eye is a fairly unique theme for a Sans fight that still manages to carry the same kind of intensity while showing you that THIS Sans wants you DEAD.
CHEETO THUNDER, aka the theme for the secret boss of Chapter 3 is a fun and intense theme.
Base-Breaking Character: While there were many Deltatraveler fans who enjoyed GG!Underfell's Sans's character, many Undertale fans were not a fan of how he was handled in the section, deeming his slip into psychopathy and his lack of humor as character assassination, given that GG!Underfell is supposed to be a canon divergent version of the original Undertale. On the other hand, there are other fans who believe that there are reasons for him losing it in Deltatraveler, most involving how he specifically has a grudge against Kris, Susie, and Noelle.
Big-Lipped Alligator Moment: You can encounter Ralsei smoking a fat blunt in the area where you find the Toy Knife, only to reveal it was a shared hallucination. The event makes zero sense, and isn't referenced again for the rest of Section 1. Averted in that he does show up again on the normal route of Section 2, though he questions if there's any point in him being there.
The final boss of Section 1, Flowey. He's definitely a welcome sight to those who found the section being too easy, as unlike the other enemies, he's actually strong enough to put up a fight with both you and Susie, having fast paced attacks that keep you moving, a sick ass boss theme, references to the end of Undertale's neutral route, and showing the player that some enemies have to have their health depleted even if you're on the pacifist route. While it obviously would be overshadowed by later fights, this depiction of Flowey is undoubtfully memorable.
Hard Mode ramps it up even further by making his attacks faster and covering the bullet board with spikes in one of his attacks. And you can't use Red Buster since Kris is in Flowey's clutches.
The final boss of Section 2, Porky Minch. You need to topple his mech's legs to make it vulnerable, and you also need to master the new cyan SOUL mode, which adds a parry system — the two mechanics even come together, with parrying being a feasible way to topple the mech. Not to mention that you have access to Paula's PSI, allowing for even more options to fight with.
Section 2's Obliteration Route has a similarly great finale. Ness and Paula themselves challenge the Delta Warriors, with their PSI and normal attacks creatively integrated into the Bullet Board mechanics. But when Ness goes down, Paula takes you on herself, her attacks becoming even more ferocious as — appropriately — MEGALOVANIA plays.
The secret boss in Section 3: Jerry. Its fight is notable for being shockingly badass, requiring that you need to pay attention to the flavor text to figure out what you need to do next and having difficult to dodge blade bullets. The violent ending is probably even more impressive, with Jerry gradually increasing its defense as its HP gets lower until you activate the SOULFinisher ACT, leading to an amazing parody of a Jerry related joke started by Shayy with Kris delivering Rapid-Fire Fisticuffs as "It Has To Be This Way" plays in the background.
Difficulty Spike: After going through the familiar Undertale enemies in the Ruins and that one unexpected Wake-Up Call Boss, enemies in later sections are much more appropriately challenging to accommodate three party members, having more difficult spare conditions, higher health, and even featuring different SOUL modes early on. Without Susie and Noelle, many of these enemies are essentially Bosses In Mook Clothing.
Game-Breaking Bug: Prior to the 2.05 update, there is a chance that defeating Mondo Mole will end up duplicating his NPC so that his living body is standing above his dead body. "Proceeding anyway" will result in the gang eternally walking in place, softlocking the game.
After resting up at Toriel's house on an Obliteration Route, Susie will give Kris the Butterscotch Pie, saying it's "a little taste of home."
After the group rest at QC's house, Kris, having had another dream about what was going on with the SOUL, hugs Susie, much to hers and Noelle's shock. Susie is initially taken aback, but quickly returns the hug, and even tries to awkwardly comfort them. Once they separate, she reassures Kris that everything will be fine, adding a request for them not to tear up anymore, or she'll start crying too.
If you aborted the Obliteration route after killing Snowy, then Susie will be the one to initiate the hug.
And on an Obliteration route, Susie will hug Kris after they have the offscreen talk.
On a Pacifist run, Snowy defuses the fight against the final Feraldrake by reassuring him that he's okay, leading to a rather touching scene
In Section 3's Intermission, Sans will refuse to buy the Papyrus Charm from you, saying, "really?? You'll just do that to him?"
Narm: In Section 2's Obliteration Route, if you push Kris too far by trying to manipulate Noelle in a similar manner to the Snowgrave Route, they will ask Noelle to freeze the SOUL and throw it far away. While the circumstances of the event combined with Kris Suddenly Speaking makes it both nightmarish and tear-jerking, the silly glass-shattering sound that plays when the SOUL hits the ground combined with Susie's reaction makes it unintentionally hilarious.
"GOD DAMMIT KRIS WHERE THE HELL ARE WE?"Explanation This is practically a given, thanks to the game being based on this very meme. Susie makes this very quote (and variations of the quote) throughout the game (unless you're on the Obliteration Route).
doobieExplanation "Ralsei's" quote upon being interacted with before he disappears. This being completely out of nowhere is inherently funny on its own.
SillygraveExplanation After the reveal that Section 7 would take place in Toontown Online, the Deltatraveler Discord made jokes about an alternate route similar to Deltarune's Weird Route, but instead of Snowgrave, Kris makes Noelle use "Sillygrave" instead.
Ryno doesn't existExplanation At some point after Section 2's release, Ryno stopped using the Deltatraveler Discord for a while. When she rejoined later, she joked that she doesn't exist whenever someone brought up her sudden reappearance.
Sad NoelleExplanation In Section 2 on a normal route, when Paula hops onto Kris' back, Noelle says she wants to piggyback too and hints to Susie that she wants to piggyback on her. Susie says she doesn't want to carry anything, causing Noelle to make a face reminiscent to the sad Spongebob meme. The talksprite extended outside of the Deltatraveler community.
"No bitches" NoelleExplanation In Section 3's intermission, Noelle's sprite for looking through the telescope looks a lot like the "No bitches" Megamind meme, causing people to combine the two.
Self-Imposed Challenge: On an obliteration route, there's no reason that you would need to threaten to kill Susie after hurting her, and if you do she will become devious and will occasionally directly sabotage the party.
That One Boss: Rather fittingly, Underfell Sans is as much of a nightmare as his canon counterpart, as he reuses many of canon Sans' attacks. Although if you are on a pacifist playthrough he won't have his retribution mechanic, that doesn't change the fact that many of his attacks have very little room for error; capped off by his final gauntlet attack, which hits hard and doesn't give the player much breathing room.
That One Attack: In the Obliteration Route, Paula will hit you with critical hits which consist of sparks flying outward in a spiral before coming back in towards the center. It's very difficult not to get hit by this, and to make matters worse, she'll sometimes combine it with her other attacks.
is an incredibly well-done mashup of Enemy Approaching and Rude Buster (which is retooled to the more chiptune-esque instrumentation of the former).