Reading critical reviews written by the greats about their art is an extraordinary privilege. They are not suitable for didactic purposes, nor should they be read with any degree of religiousness, but rather I think they ought to be regarded as fine exercises in sensibility. With the likes of Debussy, that sensibility is most likely top-notch, and thus there is always hope that a glimpse of personality is detected in between the fluff.
Since these reviews were merely short musings to be published in La Revue blanche, they are not thorough or consistent in the least. So the main interest gravitates towards little vignettes Debussy says about art on the whole and about the art of the composers under criticism.
Debussy seems to champion a very typical attitude towards the music of his time: it's all too derivative (mainly of Wagner), it sacrifices emotion over technicality and it is superabundant in structure. Though a great admirer of some aspects of Wagner, Debussy seems to be veering towards the simple and honest, and in his eyes the only true way to achieve beauty is to connect with the true nature of Art. This cannot be achieved with perfectly machinated leitmotifs, incongruous relationships between the libretto and the music, or by way of imitation (either of other musicians or the nature). No, it has to be something more magical, something that Debussy thought genuinely to exist between the human souls and the boundless soul of Mother Nature, that only art can reveal. And he wanted everybody to be able to partake in this rapture, not simply those in the haute monde or in the know.
What Debussy also clearly thought was that music conveys the personality of the composer. He could write very engrossingly about how music exposes Beethoven's superb humanity, Mussorgsky's parsimonious exhilaration in discovery and Franck's simple Christian kindness. Or how it brings to light Gluck's staid compromises or Wagner's megalomaniac pomposity. These are the most fascinating tidbits of all, since they showcase the stellar sensibility that had graced Debussy in conjunction with his musical expertise. (I think part of what made him dislike so many "architectonic composers" was that he himself was so expert in figuring out patterns and techniques behind music, and, being the musical revolutionary he was, he wanted away from it all after too much familiarity.)
Debussy was also entranced by what he called "colour" in music, instancing the likes of von Weber and Richard Strauss as the ideal aural painters. I cannot quite put my finger on what is meant by this colour - only vagues ideas flit through my head - but it seems to be that Debussy himself was a master at this compositional brushwork. Everything he does is woven into this perfectly harmonious stream of sounds, where dissonance loses its ugliness and consonance its obviousness. His chords glisten solar-white and his harmonic progression undulates with its deep, dead blue; there isn't a hint of black in his masterpieces (very impressionistic of him, indeed!), for even the shades are merely of deeper chroma of the primary colours. So in Debussy's work, this "colour" would be harmonic richness joined with careful sensitivity and spellbinding uniformity of texture.
Though Debussy loved music and nature first and foremost, he was not blind to the extra-musical aspects of concerts. He was quick to notice the appearance and demeanour of the conductor and how his behaviour affected the orchestra. He mused on the importance of the milieu where the music is heard - and was ruing the fact that the great outdoors was not utilised to its full extent. Nor could he refrain from mocking those, who only came to these gatherings for their social import. Threadbare things, but it's somehow more insightful when they're said by a great composer who lived a century ago.
In sum, Monsieur Croche is an easy and inspiring read, but I can't say it catered to my expectations that well. I was expecting to hear a bit more about Debussy's method of working, instead of him complaining about his shabby contemporaries and the vanity fair that generally revolved around him. Still, he wrote well and lucidly, and the simple fact that one can read Debussy is enough to make it all worthwhile.