Here is a lovely illustrated essay on the waltz by attorney (yes, you read that right) George M. Wallace of the blog A Fool In The Forest titled "Bearing Valse Witness" that's well worth your time reading. An excerpt:
Does anyone still compose a waltz? Well, the form did not die, certainly, even as Ravel was busily vivisecting it. To the north, in Denmark, Carl Nielsen was busily constructing the first movement of his Symphony No. 3 (aka the Sinfonia Espansiva) around a grand, driving waltz theme that recurs at intervals, over the objection of the sections around it. (Some enterprising choreographer could construct a fine dance from that movement, if not the entire symphony.) Nielsen composed his symphony in 1910 and 1911, conducting the premiere in 1912. His waltz, therefore, falls in the middle of Ravel's composition process: begun after Ravel started his Valse in 1906 but completed prior to the outbreak of the war that so influenced Ravel's final version. While rumors of war can be detected in the brass and percussion — they become explicit in Nielsen's 4th and 5th symphonies of 1916 and 1920-22 — the Espansiva is a fundamentally optimistic piece, particularly in its final two movements.