As someone who was very recently not interested in Haydn (or most music from the Classical period, for that matter), I feel compelled to give you my take on his music. About 2 years ago I was encouraged to play a Haydn sonata movement, and ever since his music has grown tremendously on me. I believe that your reaction might be fairly typical for someone who hasn't looked at his music in depth. What you said about all of his music sounding similar can be somewhat true, but only on a surface level where you haven't even scuffed the floor! Here's probably why all of his music sounds similar to you:
If you're looking at his music from a theory perspective, almost all pieces written in the Classical period follow the basic harmonic formula of I-V-I. There was almost none of what we today would consider creative handling of dissonance. At this point, music was moving away from the more "complex" multiple voices (polyphony) that's characteristic of music from the Baroque period and towards a clean melodic line accompanied by subordinate harmonies. The sparseness of Classical period harmonies and a more orderly form may be what's causing you to label everything as similar.
BUT!!! There is plenty of room for beauty, brilliance, wit, and music
to emerge in Haydn's works (even though he doesn't pull a Prokofiev and conclude sonata movements with unresolved leading tones
). In the Haydn sonata I'm working on right now (XVI: 30, A-major), the opening is much like a court dance, followed by a fanfare of horns heralding a hunt, followed by the stacatto plucking of mandolins, followed by another fanfare of horns and an adagio for the crowning of a prince. I find that most music tells wonderful stories like this, and Haydn's is no exception (or maybe I'm just crazy and imagining all these stories, but it sure is fun for me!). Here are some of my favorite Haydn piano works:
Piano Sonata in F-major, Hob. XVI 23, Adagio- Easily my favorite piece of music by Haydn.
Piano Sonata in A-major, Hob. XVI 30- What I'm working on right now.
Piano Sonata in c#-minor, Hob. XVI 36
Don't give up on Haydn so easily. Dig deeper. And enjoy it for the brilliant music that it is!
Sorry for the rant,