During the 19th century very little of Haydn's music was played. After World War II, it was suddenly realized how significant a creator Haydn was and how wonderful his music really is. His greatest technical contribution was his consolidation of sonata form. In his early years he wrote tuneful music with little development, arias for instruments, as it were. But as he developed, he worked out the sonata principle better than anybody else in Europe.
Sonata form is essentially contrast and development.
After the themes are put through their paces, the recapitulation brings back the two original themes, which rounded out an arch. Haydn invented none of this, but no composer in Europe refined the principle better. All of Europe knew him for the master he was. His choice of keys is even more adventurous than Mozart's, although Mozart's is more adventurous within the key. Haydn's later music contains even more anticipation of romanticism.