The same question that was troubling me at one time might be troubling you now. I once wrote a letter to a newsgroup, asking something like, "Why is Mozart better than Salieri? And why is Beethoven better than Dittersdorf? When I tune in late on the classical station, I find that I can't tell Mozart FROM Salieri. And I can't tell Beethoven FROM Dittersdorf. Do I revere Mozart and Beethoven just because everyone else does? I feel guilty for being hypocritical."
Someone wrote back that Mozart and Beethoven were innovative composers, and a composer's innovativeness cannot always become evident in a single hearing.
I have the score for Mozart's piano sonatas, and I've read a couple of books analyzing the Mozart piano sonatas. So here are some cases in which Mozart stepped off the beaten path:
--In the F major sonata, K 280, he wrote all the movements in sonata form, not just the first movement. Mendelssohn did the same thing in the Scotch Symphony.
--In the D major sonata, K 311, he wrote the themes in reverse order in the recapitulation. This puts the first theme at both the beginning and the end, making a nice pair of bookends. Many Twentieth Century composers followed that practice.
--In the A major sonata, K 331, he wrote a theme and variations in place of the usual sonata movement. Beethoven did the same thing in his Ab major sonata, op. 26.
--In many of his sonatas, he wrote more than two themes in the first movement. Beethoven followed suit in his symphonies.
--In the C major sonata, K 545, he wrote the first theme in the recapitulation in the subdominant key in place of the usual tonic key. Schubert did the same thing in his earlier symphonies and in his Trout Quintet.
--In the Bb major sonata, K 570, he wrote two themes in the first movement which were similar to each other. Beethoven did the same thing in the Appassionata sonata.
--The c minor sonata, K 457, is believed to have inspired Beethoven's "Little Pathetique" Sonata, op. 10 no. 1, and his "Pathetique" Sonata, op. 13, both in the same key.