When Beethoven wrote in A♭ major he knew it was the key of the grave — and knew his audience knew it too.
Affective Key Characteristics
The association of musical keys with specific emotional or qualitative characteristics was fairly common prior to the 20th century. It was part of the shared cultural experience of those who made, performed, and listened to music. When Mozart or Beethoven or Schubert wrote a piece in A♭ major, for example, they were well aware that this was the "key of the grave," and knew that many in their audiences were as well. We lose a part of the meaning of their music if we are ignorant of their affective choices.
Although these characteristics were, of course, subjective, it was possible to conceive of each key as unique because each key actually sounded distinct within unequal temperaments. When equal temperament became the dominant tuning after 1917, the aural quality of every key became the same, and so these affective characteristics are mostly lost to us.
One of the most influential descriptions shared in German-speaking cultures in the late 18th and early 19th centuries came from Christian Schubart's Ideen zu einer Ästhetik der Tonkunst (1806). Each key page below carries his characterization, in Rita Steblin's translation (A History of Key Characteristics in the 18th and Early 19th Centuries, UMI Research Press, 1983).
Major Keys
C Major · D♭ Major · D Major · E♭ Major · E Major · F Major · F♯ Major · G Major · A♭ Major · A Major · B♭ Major · B Major
Minor Keys
C Minor · C♯ Minor · D Minor · D♯ Minor · E Minor · F Minor · F♯ Minor · G Minor · A♭ Minor · A Minor · B♭ Minor · B Minor