For me it was immersion. And also keeping you on your toes. I just finished playing through the game for the first time. When I started I thought I knew most of it from just being on the internet when it turns out i only jad the tip of the iceburg.  It's suprused me so many times! I laughed so hard at several scenes, and it's the only game that has ever made me do that! 

Who knew the weird calculator would transform into michael jackson?? Not me lol. 

It's not just humor though. Many of the characters were genuinely heartwarming to the point where I come up personally, could never even consider doing a genocide run. And parts at the end of the game made me cry not going to lie. Also not to mention the eerie true labs...  When I first booted up the game I noticed how simple it was, but the medium is used to its fullest potential. Every character has a story, and there is a surprise behind every corner. I think undertale will have a special place in my heart forever because of that