I'd have to say Mr. Hooper's death really did me in.
I am sure that dates me a bit, but watching Big Bird getting sad and confused, and Maria and Bob welling up with real tears. Heavy stuff. As a kid I think it's the first time I had death explained to me by adults without any fluff or BS. He isn't coming back "because." Gordon told Bird. " Just because?" Bird replied. "Just because..." said Gordon.
I think it's also probably the first time I'd seen a grown man cry. The writing was brilliantly crafted. They started such a heavy conversation with Big Bird giving his art to the human cast, portraits that he obviously worked hard on thinking about each of the different people and the reactions they would have when they were given these works.
As an artist that gave me an extra punch in the guts, creating personal, custom work for someone, who will now never be able to experience it. And the not mentioning Mr. Hooper by his name till the very end of the scene when Big Bird screws it up one last time calling him "Looper" to be corrected by Maria through her tears, the decades long running gag coming to a close. Heavy stuff...
The camera pulls back as the whole cast gets up to hug and console big bird, it keeps pulling back, giving them a moment alone to hug, fade to black... I was left sitting in my parent's living room with a half eaten peanut butter and jelly sandwich clenched in my hands. My little kid mind reeling as I had to try take in what had just happened on the screen. This ain't Barney kids. Brilliant, more programming like this please.