Scary intimidating music
Fans of classical music absolutely despised them (sometimes) for "butchering" various well-known pieces in their attempts to interpret these standards in a rock idiom.
There is a nugget of truth to this, of course (Pictures at an Exhibition is often quite a stretch from the original Mussorgsky piece, to put it mildly), but ... I'm sure that a good number of the band's fans became fans of classical music due to their efforts, so that should be worth something.
In the Court of the Crimson King, regardless of all of the mellotrons, was extremely guitar heavy, not to mention the saxophones and other reed instruments.
Regardless, however, this is a perfectly legitimate statement on the general level.
Fans of punk have always detested them as they would any prog band, but this extended far beyond normal levels of loathing; as an example, one of the staples of late 70's Sex Pistols shows was to burn life-size statues of Keith Emerson in effigy (or so the legend goes, and I really hope it's true because it's a hilarious legend).
Or maybe the older generation was just mad about all these young whippersnappers infiltrating their societal niche ....
(AUTHOR'S NOTE): There used to be a paragraph here where I made some asinine statements about the borders of what exactly constitutes prog rock.
ELP, however, was "pretentious" and progressive from the very beginning, which makes sense.