There was a study of #1 songs a few years ago and they found that these songs all shared the following characteristics:

1. Genre: Pop or Country

2. Subject Matter: Romantic, sad, heartfelt, humorous

3. Lyrics: Tells a clear story and/or relates a strong opinion

4. Narrative: First person or second person

5. Melody: Linear melody in verse and soaring melody in chorus

6. Harmony/Chord Progressions: Few chord changes in verse and (significant chord changes in chorus

7. Song Structure: v, c, v, c, b, c, out

8. Tempo: Mid- to up-tempo

8. Time Signature: 4/4 time

9. Intro: No longer than 13 seconds

10. Songwriters: Co-written between artist and another professional songwriter.

This study must have taken place a number of years ago (note that the genres represented don’t include hip hop songs), but regardless it is instructive.