Awesome Music: There's a reason that this is one of the best-selling albums ever.
Ear Worm: "Money". It's arguably the most famous song to feature an unusual time signature (in this case, 7/4), led by an memorable bass riff. The switch to 4/4 for one of the best guitar solos of David Gilmour's career doesn't hurt, either.
One-Scene Wonder: Clare Torry's wail on "The Great Gig in the Sky", entirely improvised by her. (She was initially paid a flat studio fee of just £30; she later sued for a portion of the royalties, and settled for an undisclosed sum. The song is now credited to both her and Richard Wright.)
Shocking Swerve: For all of their psychedelic experimentation on previous albums, "On the Run", a slice of proto-trance music in the middle of a rock album, is still this trope. Doubly so for listeners in North America, where many heard the band for the first time either through this album or through the success of the rocker "Free Four" from previous album, Obscured by Clouds.
Some Anvils Need to Be Dropped: Quite a few songs on the album qualify, but "Us and Them" really takes the cake. Its message is simple, universally applicable, and as subtle as a hammer to the face: Everyone is capable of being inhumane, and trying to dehumanize your supposed enemies as some monolithic "Them" has never made the world a better place. Put all that together, and you've got one of the best protest songs in the history of rock n' roll.