But I can see what bothered Debussy. Calling his music 'impressionist' really doesn't tell you a thing about it. Debussy is a very subtle composer who falls exactly on the transition from 19th century romantic music to 20th century modern music--in fact, he had a lot do with with making that transition. Debussy is often described as the most influential composer on 20th century music. In 1909-10 from December to February he wrote a book of 12 preludes for piano and followed this with another book three years later. In this he is taking his place in a long tradition starting with Bach's Well-Tempered Clavier (24 preludes and fugues in all the keys) and continuing with Chopin's 24 preludes, also in all the keys, but arranged differently from Bach. Debussy's preludes are not in all the keys. In some it is hard to be sure what key they are in. Also, Debussy gives descriptive titles to his preludes, but these titles appear at the ends of the pieces. So if you are a pianist playing through, first you play the piece, then you see the title.
Here is one of my favorite preludes:
That odd, halting figure that we hear first accompanies us throughout. The melody comes in short, breathless phrases. The harmony is complex, but however far it wanders, that halting figure, rising from D to F, never varies. Towards the end there is a stream of chords that seem entirely apart from the key, D minor. Then the piece closes with a plagal cadence: G to D. What a mysterious little piece! "Footsteps in the Snow".