Doing this stream of consciousness style. Thanks, @nathanveshecco!
  1. Unexpected key changes
  2. When a song by a solo performer gets remixed as a duet
  3. Gospel choirs
  4. When a song transitions over into the next track but you can't tell
  5. Harp in popular mysic
  6. Women singing low
  7. Men singing high
  8. Men singing low
  9. Women singing high
  10. When rappers sample songs and put a new beat on them
  11. Off beats
  12. Double entendres in lyrics
  13. Spoken word sections
  14. Covers in new musical styles
  16. Musical pauses in the middle of a song
  17. When a song changes styles partway through
  18. When a musician is plain talented, regardless of instrument or musical style
  19. Fusion of genre
  20. Vocoder
  21. Intentional autotune
  22. Any other vocal distortion techniques
  23. When artists innovate their musical style
  24. Speak-singing
  25. Whisper-singing
  26. Synths
  27. When the person singing sounds like their on the verge of crying
  28. When the singer does a little chuckle
  29. Acoustic renditions
  30. Using non-drums as percussion instruments
  31. Long/unconventional words in lyrics
  32. Singing with just an accompanying drum beat
  33. When a singer starts without backing music
  34. Songs that end abruptly