In Schubert: A Musical Portrait Alfred Einstein goes into considerable and admirable detail on the meaning of moderato in Schubert: it concerns, in his words, a "lyrical quality." He points out that Beethoven never used the term. (Haydn, on the contrary, used it very, very frequently; it could be that Haydn was Schubert's source for the sense of the word.)
Never, except for the Op. 17 Sonata for natural horn and piano, which I have been playing with great delight together with my son. The sound of the horn is instructing me why Beethoven would use that term for two of the movements in this piece. The sound is itself so lyrical, so not homogenized, that it requires special timing, special attentiveness. To play the work as if that is not central to its concept is to do it a great disservice. In fact, it renders the piece trivial, which is why so few people even know (about) it and those who do turn up their noses at it.
