These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Most Smiles that appear as mid-bosses are usually this, since they normally require a single hit to kill. However, most of these are justified since they become regular enemies. The last two do not, however, and are two of the toughest bosses in the game. In fact, you can make an argument that the last one is the Final Boss and the bosses after are Post Final Bosse.
Awesome Music: The soundtrack covers a wide variety of genres, and ranges from creepy and atmospheric to just plain awesome and catchy.
Some good examples include Sweet Blue Flag (Ayame Blackburn's battle theme,) Tecks Mecks (the stage music for Cloudman,) Rave On (the most awesome music for walking down a completely featureless corridor ever) and Blackburn (the stage music for the inside of Curtis' house.)
Even more good examples include Emoticon (Suzie's theme) and Residence (one of the stage themes for Alter Ego.)
Complete Monster: Curtis Blackburn's first appearance is a cutscene in which he kills a prostitute in his car before walking into an office and killing every person in it for no apparent reason. Then you find out he killed one of the player characters and controls the female half of the black market for orphans (who are used for organ harvesting). Next, Curtis gets revenge on his partner (who controlled the male half) for cutting into his orphans... by raping and killing his wife in front of their son, killing his children, and showing him his daughter's severed head before killing him. A pedophile with a fetish for anime loli, he harvests organs for the US government from little orphan girls he molests by using a giant car-wash abattoir hybrid, that rids the body of all organs, cleanses it and closes it up, then leaves just a shell of the dead body. After the boss fight in the room outside you can see naked little girl hollowed out dead bodies.
Cult Classic: Introduced the Western world to Goichi Suda, and many still consider it the high water mark of his career.
Ensemble Darkhorse: For a character that gets very little development, Con Smith is notably popular among fans of the game.
Con is likely an inversion of Tier-Induced Scrappy: he's very fast at shooting, reloads quickly, runs quickly (so you can get to Harman's Room and back in a short amount of time), and the low power of his bullets and low health don't matter because you can take out a Heaven's Smile before it gets to you.
KAEDE has literally zero ingame character development, outside of the Camellia Smile symbolism, but remains popular for her very simple, yet strikingly memorable, character design.
Andrei Ulmeyda, despite only appearing for one chapter, is a fan-favorite thanks to his over-the-top eccentricity and incredibly hammy performance by Cam Clarke.
LPers Chip and Ironicus really like Travis and Yoon-Hyun during their LP of the game.
Goddamn Bats: Broken and Rolling Smiles move really fast and their weakpoints are a nightmare to hit.
That One Boss: Ayame Blackburn is ridiculously hard to hit and has a tendency to tag you with bullets every few seconds.
Andrei Ulmeyda starts the fight by trying to horrify the player, causing them to run away from him and get lost in the maze. And running into him is a one hit kill. Good luck.
The last two Queens: the Timer Smile and Galactic Tomahawk Smile, to the point where the latter is more of a Final Boss than Greg Nightmare, Emir Parkreiner, and the Last Shot Smile could ever hope to be.
That One Level: killer8 makes the Kaku Building into the hardest level in the game, as the building has very tight spaces while dealing with the Smiles, who one hit kill most characters.
They Wasted a Perfectly Good Character: 7 playable characters (plus Harman making 8), but only three of them (Garcian, Harman and Dan) get any real focus. Mask gets some interaction with Jean, Con gets a couple of lines in a cutscene, and KAEDE, Coyote and Kevin are just left by the wayside.
And related to that is Ayame Blackburn and her animegao mask.
The Untwist: Garcian's identity and his murder of the other Smiths isn't just heavily foreshadowed, it's everything but flat-out stated long before The Reveal. Since this is a Suda51 game, opinions are divided on whether this is a poorly-executed Twist Ending, a parody of twist endings (a la No More Heroes), or a deliberate untwist.
But perhaps there is a hint of twist, since Emir apparently shot himself after killing the Smith Syndicate on the rooftop, meaning that he was dead, too.
It's a case of All There in the Manual. Hand in killer7 implies that Emir has been reborn. Harman even says in the game that Emir was not a mortal being.