In spite of all the jokes and various antics (which are mostly very amusing and charming!) there's still a pervasive feeling of bittersweetness that flows throughout the game, especially on my first playthrough where I accidentally killed the first boss. The thing I remember the most about that first playthrough was the loneliness of the trek through the underground, of meeting all these nice, lovable characters and then leaving them behind. When I saw the scene at New Home, it made me cry partly because of this loneliness, and partly because I felt guilty for the death I'd caused at the beginning of the game. To me, this is really the "true" Undertale experience, and I think too many people worry about doing a 100% pacifist run on their first playthrough, even though the game definitely sets you up to fail at it at least once...

Also, the OST still fills me with a powerfully wistful feeling. Toby Fox inspired me to start writing music myself, and now I'm pretty good at it, so I have a lot to thank him and this game for.