So far, 2016 is turning into an exceptionally awful year when it comes to rock musicians passing.
Emerson was THE most innovative keyboard player in rock history; for one thing, he was the first to show that the Moog synthesizer could be used in live performances (back when you had to patch together modules to make sounds and could only play one note at a time). His technical facility and ability to fuse serious music with rock were second to none, and he was without doubt the rock keyboardist who played with the most showmanship. His sense of melodic invention was uncanny (the synth solo in the original version of "Tank" sounds like a sax solo from some post-apocalyptic future (go here: https://www.youtube.com/watch?v=i-VtGV3FnF0 — the solo begins at 4:14).
I was lucky enough to have seen ELP live three times: in 1971 at the Hollywood Bowl (with Edgar Winter's White Trash and the original Humble Pie; you really got your money's worth at concerts back then), in 1972 (with the original Mahavishnu Orchestra as the opening band!) and in 1977, at one of the few gigs they did when they still had the full orchestra and choir with them. BTW, the sound system at that concert consisted of around 24 Clair S4 cabinets flying on each side of the stage. Each cab was all-JBL: 2 x 2240s (18"), 4 x E110 (10"), 2 x 2440s on custom horns and 2 x 2405s, all arranged vertically in the cabinet so that when multiple cabinets were deployed they formed long lines of drivers that controlled vertical dispersion. It was absolutely the best-sounding concert I had ever attended up till then (my first was in in 1967) and maybe of all time.
At all three concerts they managed to kick serious ass while playing serious music. RIP, Keith; you will be missed.