Pro Amazing characters
The characters in the game are written well, each with their own unique personalities. Some may be too shy, and will lower their hit points when you attack them in battle, just to avoid offending you. Others are deeply caring and do their best to help and protect their friends and family, including you as the player. Many of the characters are quite funny in their dialog and the unexpected choices they make throughout the story, including the ways they react to your choices. Almost every character is incredibly memorable and enjoyable to get to know better.
Pro Your choices lead you down different story paths
Depending on your choices, your playthrough can change in drastic ways. If you don't want to kill enemies in battle, you can choose to have a dialog with them instead. Doing this consistently is basically a pacifist playthrough where you don't kill anyone, and characters respond to you in certain ways based on that. Or you can go for a genocide playthrough, which is basically what it sounds like, where you kill everyone you come across whenever you get the chance. This can have serious consequences, since the game does its best to make you feel bad about this approach, by having certain characters hate you. It's an interesting way to get to see the world and story from different angles.
Pro Incredible soundtrack
Just like the simple but pleasant graphics in the game, the music matches this with a retro-sounding score that fits right in with the art style. There are a lot of catchy tunes that only use a few sounds to make up memorable songs. The simplicity really adds to the charm, with some of the songs easily able to get stuck in your head for days at a time. It's nostalgic enough for older gamers who grew up in the early eras of gaming, while still having enough to pull in newer gamers who aren't as familiar with games from the 1980s and 90s. There's something here for everyone.
Pro Many interesting secrets to find
There are many secrets to discover in the game, like hidden mechanics that might take you by surprise. It may take a couple of playthroughs to find them all, but once you do, it may completely change how you see the story, the characters, and a lot more. There's a lot more to Undertale beneath its simple-looking surface.
Con The visuals could be better
Much of the game is plain-looking with simple graphics, but sometimes the visuals are just awful. The pixel art can be charming for some players, though it's hard to ignore when certain areas are barren or have ugly textures. It's notable that a single person created the whole game; it would have been nice if they'd had more time or resources to clean up some of the visual problems.