I'm not talking something huge in a song, but something small that you might miss the first time you hear a song, but when you start to break down the bass line, something makes you say "cool". Example - Boston's Rock and Roll Band. The verse is a basic riff going from the E to the A chord, but the 4th time through, there is a chromatic run back into the E chord. Subtle for a song like that, but nice. Others?