TP, great post.
I started watching the Celtics right around 2001, when they started putting things together under O'Brien. The '02 season was the first where I watched most of the games. Boy, was I excited to watch this young team that made the ECF grow and get better. Oops. But as disappointing as the '03 season was, I remember the '04 season being brutally disappointing for young me. They were just so bad. Antoine was gone. O'Brien quit. Nothing went right. Getting bounced in the first round with just 36 wins was disgraceful.
After that, as far as seasons I've witnessed, the '13 season was really disappointing. I thought they still had another run in them. I thought Rondo was going to take over. I was Jeff Greened. The funny thing is, this is one of my favorite Pierce seasons, because after Rondo went down, he put the team on his back and carried them into the playoffs. But that was definitely not the season I thought it would be.
Those are the two big ones, for me. Maybe there's someone here who can talk about the '78 season?
I'll wait and see what happens in the playoffs this year.