There were lots of missteps the 1980s Celts made in hindsight...
1) They drafted horribly. After Kevin McHale, Reggie Lewis was the only draft pick that became a solid contributor for the team.
2) They traded Gerald Henderson, and took Len Bias with the draft pick they got in the deal. This should have been an all-time great trade, but in hindsight, it hurt.
3) They let Kevin McHale play on a broken foot in '87. It turned out to be all for naught as the Celts lost in the Finals, and it hurt McHale's career.
4) They traded Danny Ainge for Ed Pickney and Joe Kleine. The window was pretty much shut at this point, but trading Danny didn't help.
I agree with others that Bill Walton was essential to our title run in '86. With Max in place of Walton, it could have closed the window one year earlier.