Undertale's base mechanics require some deft dodging skills, so be ready for that.
The game has a morality system, and based on how you play, the game completely changes. Almost all your interactions will be altered--as will the ending. Not only that, but the game remembers actions you've done--even if you save over them.
Make sure you mark spoilers if you want to talk about them.
And how could I forget--this game has some amazing music!
Seriously
This music
Is Great!
P.S., if you want to link music from this game, please link from this youtube channel, since it doesn't have any images attached to the music (i.e., no spoilers!) Also, if you read the comments and then complain that they had spoilers, you're really dumb! Don't read comments if you don't want spoilers!