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?
Basically everything Jeff Ament does. The dude is king of subtle but amazing fills and bass work in general. Just going absolutely nuts for the first verse. I mean certifiably insane licks. The rest of the song is a little mellower, but man these guys rocked so hard. Legends and idols man.