Barely 10 seconds into the movement, Beethoven makes an ominous move through D-natural coming to rest solidly on C-sharp (Ex.3), a tone far removed from the tonic. While it is not overtly dissonant, with this unorthodox tonal shift, Beethoven creates a brief moment of uneasiness, a foreshadowing of the tension that will characterize much the first movement.
Writers are fond of making comparisons between Beethoven and Mozart. In the case of the above passage we need not speculate how Mozart would have treated it, we already know. He used the same theme in his overture to 'Bastien et Bastienne'. Click here for Mozart's treatment of the first subject theme. Notice there is no drastic deviation from the tonic and no sense of foreboding.