Nelson was wearing denim, long haired so to speak, and was playing folk/country material he worked up with the Stone Canyon Band. Well, the crowd and promoters of this charity event expected 1950s music being performed and Rick didn't do much of his 50s stuff - he did a controversial cover of the Stones' recent hit "Honky Tonk Women" The crowd for the most part booed because they wanted the Nifty Fifties.
It was the dawn of the Great 50s Nostalgia Phenomena that spawned interest in and success of "Happy Days" and "Grease"...it can all be traced back to October 1971 when Rick went to a garden party to reminisce with my old friends....
(BTW, if you get a chance, see Rick's "Hello Mary Lou" as performed on Saturday Night Live in Feb. 1979. It is an excellent live rendition)
re: Elvis...3 concerts.
1) MSG 72 Sunday matinee with a presspass provided by WABC
2) Nassau Cty Coliseum, June 22, 1973 (Elvis lived in Uniondale, NY for 3 days in '73)
3) Nassau Cty Coliseum, July ?, 1975 (can't find that stub)
I know a guy who saw Elvis, 66 times in concert.
Sixty-six times is most impressive. That is dedication to being in presence of The King as often as possible.