Maybe it would be good to say why you thought Copland was such a great composer first. I have to say that he's probably my least favourite of what I think of as the five biggest names in American composing: Barber, Ives, Copland, Gershwin and Carter.
In my my mind, the first two and last two fall nicely into pairs - Barber the consummate professional and master of traditional technique opposite Ives the "amateur" with his striving transcendental experimentalism, and Gershwin the popular, melodic and sentimental against Carter the dissonant, spiky, phantasmagorical "musicians" composer. Copland is somewhere in the middle of these pairings, but actually not as good as any of them I think. I love some of his best work (Appalachian Spring, Piano Sonata, Dickinson Songs) but too much of the oeuvre seems to be significantly below par considering that he is now almost universally seen as one of the Great Composers of the 20th century.
Cage and Feldman are very dear to me also but are far too strange/individual to seem in any way reflective of some kind of national aesthetic.
It's difficult to put into words how I feel about Copland. There is something in his music that resonates with me at a very deep level - it evokes nostalgia in me for places and times that I have no experience of and does so very powerfully and convincingly that I can only conclude that Copland was a kindred spirit. It's the same feeling I get from Vaughan Williams, though with RVW I can at least explain some of it away given that we are both English and I have some of the same connection with the national mindset and landscape etc. Of course this is the wishy-washy aspect, which as I say is difficult to express in a way that would be meaningful or understandable to someone who doesn't feel the same way. I'm sure everyone has a composer or two that 'speak' to their soul, and Copland is one of mine.
Subjective assessment aside, I absolutely feel Copland is worthy of, as you say, a
Great Composer tag. Everything you've said about Barber, Ives, Gershwin and Carter could equally apply to Copland. Listening to the early songs last night and thinking about how mature (mature as in confident, not as in sounding like the mature Copland) and assured they sound and comparing this with the early orchestral works, such as
Grohg , in which we can hear Copland absorbing the music of Europe at the time (Bartók, Stravinsky, Schoenberg) but molding it into a unique, masterful composition, as colourful and spectacular as anything those three composers wrote.
And then the
Piano Concerto, which only a few years down the line from the songs already show a vast talent and imagination at work and, within it the seed of the familiar Copland known to many.
I feel that perhaps overexposure to the big ticket works such as
Billy the Kid and
Appalachian Spring has dulled peoples appreciation for actually how important and wonderful they really are. My exploration of Copland's music both earlier and later than the familiar Americana works has really enriched my appreciation of Copland and how wide-ranging and often exceptional his oeuvre really is. It seems to me that Copland was a jack of almost all musical trades and a master of many.
I really hope this thread will lead to the same revelation for others by providing a bigger picture, the full context and, with any luck, encouraging you to listen along and enjoy.