> Graphics: I mean... pixels. The game doesn't need to be all that 3D and pretty and stuff to execute a good story. To be honest, it's very easy to underestimate it at first glace. Personally, when I started playing it, I went, "Eh, I can name other pixel games better than this." It was only after I finished a playthrough did I regret what I said.
> Music: The music used is really gorgeous, and what I love about it is that some of their music is just recycled. Dummy's, Napstablook's, and I believe Muffet's battle music all sound the same, just with different pitches and timing to match the character. They made each character a unique music without having to think of new, different sounds all the time. Even Megalovania was recycled, but the music matches Sans perfectly.
> Characters: Oh man, the characters are awesome. Through neutral and genocide runs you don't really get to know the characters very well, but the pacifist run makes sure you actually interact with each of them.
What I really, really, really like is the unexpected twist with Sans; you see him as this funny, joke-loving guy in the neutral and pacifist, but hot damn I did not expect him to be the final boss in genocide run. He's been keeping lowkey all the time and then just suddenly BAM he kicks your ass in genocide without much effort. This is the reason why I really like Sans above all the other characters.
There are also the small, subtle efforts to give more flavor to the characters in the pacifist run. At some points you can call your phone to talk to Toriel and company (they are so hilarious), the snowflake monster has this slightly dark backstory with its dad and amalgam mom, Napstablook and Mettaton used to be bffs oh my god, and plenty others that I probably forgot. Anyway, I love how even some of the not-really-important NPCs are given some small story or personality to show that they're not just in the game to, you know, exist.
> Plot: The plot is the best oh my gosh. I'm pretty sure, unless you cheated or knew beforehand what to do, the game was designed to give you the neutral ending first, because you're just getting a feel of things. I love that the pacifist and genocide routes were created, they give you two different stories: Asriel's and the game's backstory (pacifist) and why Flowey is constantly destroying things (genocide). This is a ploy to keep the player curious and go through all means just to understand the game; it's not going to spoon feed you the whole thing in one go. The multi-endings aren't there just for show, they actually leave you with something to think about and the determination (hehe) to learn more.
I love how the game likes emphasizing how Asgore is the final boss, yet he never is in any of the runs (neutral was Flowey, pacifist was Asriel, and genocide was Sans). It catches you off-guard and makes you believe everything's done when there's still more. The story basically rides on the "but wait, there's more" catchphrase, and it once again sparks curiosity among players, encouraging to play again.
The pacifist and genocide run are also very tedious to do; pacifist let's you try and survive lots of enemies with a level 1 and no intention to fight back, while genocide run counters keep increasing per area. It's the question of "How much do you really want to see the end of this? How determined are you to find out more?" The making of this gameplay is really interesting, and I can see why it's done so.
Lastly, I know how frustrating it is how the game remembers your choices, but that's the truth and beauty of it. There's no resetting, there's no turning back, especially in a genocide run. Sans remembers how many times you've come back to fight him, and the game definitely knows you've done a genocide run. It's like real life -- once you commit something grave, it's not really that easy to just hit "reset" and let it erase your problems.