Amazing mastery of keyboards and composition.  Sure he "borrowed" the works of other composers, particularly classical, but in his re-orchestration, brought new life into those classics.
How ironic.  I have been listening to them lately and just received a UK pressing of Brain Salad Surgery.  I will definitely be listening with a different view. I always liked those guys.
Keith Emerson/ELP opened the door for many of us to have an appreciation/interest for classical music. My first ELP purchase was Pictures at an Exhibition on CD which was also my first CD purchase that was recommended by an older audiophile. By the time I hit 20(where has the time gone?) over 25 classical recordings were in my music collection mostly due to ELP and TAS. Out of respect I will play my original "Pictures" disc and spin "BSS" this weekend!
ELP's "Brain Salad Surgery" was a big favorite of mine. Very sad about Keith Emerson's passing. I used "Lucky Man" and "Take a Pebble " as demo tracks to show off my system back in the 70's. 
ELP was my favorite group in high school and I managed to see them twice at Cobo Hall in Detroit. What a concert they put on, Keith was a stupendous performer. Carl and Greg did a fine job too. I still have the programs from both concerts, something I did not do for any other concerts back then. ELP was almost totally responsible for me "discovering" classical music, something that was never played in my household. Man, will he be missed ...