ELP has never really been a prog group. They have always been part of mainstream rock, attempting a radical, contemporary, groundbreaking and aggressive fusion of elements from jazz, classical and (hard) rock. Most albums also has a (bitter) sweet piece or ballad suited for Top Ten (e.g. "Lucky Man", "From The Beginning", "C'est la Vie", "Daddy"). Their latest albums (Black Moon and In the Hot Seat) very much show that they are still trying to be contemporary, even if far from so radical or groundbreaking as on the first five albums. It was quite natural that ELP was invited to perform at the Isle of Wight festival in 1970 together with The Doors, Hendrix and the Who.
Their jazz background also shows in their (especially Emerson's striking) ability to improvise and play solos. They compose themselves - often in a contrapuntal way - or reinterprete others music, mainly modern classics (e.g. Mussorgsky, Bartok, Janacek and Ginastera) and not "easy" or "audience-friendly" ones like Bach or Beethoven.
However, they always put much weight on improvising at stage. This is most clearly shown live in pieces like "Tarkus", "Take a Pebble" and "Fanfare from The Common Man". No ELP concert contains exactly the same solos, and they may have strikingly different versions of some songs.
ELP is also one of the most diverse bands in the world. Every album is different from the previous and every tune on every album is a new approach. E.g. does the first album (ELP) open with the screaming "Barbarian", shifts style and intensity completely on "Take a Pebble", before it goes on to the hypnotic and intense "Knife Edge". Then some organ and piano solos ("Three Fates" and "Clotho"?), a percussion oriented funk tune ("Tank") and finally the seemingly naive ballad, "Lucky Man."
ELP changes instrumentation (e.g., between accoustic and electric), rythym, key, level and intensity, between instrumentals and songs, between classical, jazz and rock oriented pieces or parts of the same piece, and between playing all of it themselves or having an symphonic orchestra participating (as on Works Volume 1) for more than providing a good "background."
Bjo/rn-Are Davidson, firstname.lastname@example.org