Let's Get It On is written in the key of E♭ Major.
According to the Theorytab database, it is the 7th most popular key among Major keys and the 11st most popular among all keys. Major keys, along with minor keys, are a common choice for popular songs. The three most important chords, built off the 1st, 4th and 5th scale degrees are all major chords (E♭ Major, A♭ Major, and B♭ Major).
See the E♭ Major Cheat Sheet for popular chords, chord progressions, downloadable midi files and more!