Sad to hear. He was one of my favorite Mets of all time.
There's certain things that come to mind about him. Very reliable fielder, and even though he was not really much of a hitter, he still managed to put up some respectable OBPs by drawing a good number of walks. 95 walks in 1970 was his best. Of course the Pete Rose fight, which he described as hitting Rose right in the fist with his face.
Also, his defense of Tom Seaver in 1970 when Seaver had a subpar 2nd half of the season (after a great 1st half) as the Mets fell out of the race. There was one game where Seaver had a particularly bad game (for him anyway) and got booed at Shea. Harrelson said afterwards that the fans had "short memories". Always the good teammate....sticking up for his friend.
It's fitting that he is the only Met to be in uniform for both WS championships.