Doc seems like a nice guy outside of basketball and I have no real ill will towards him, but my own take for not liking Doc is threefold:
1. He signed a contract extension in Boston knowing full well that part of that contract would include a re-build of the team. Allen/Garnett/Pierce, even if they retired as Celtics, were getting old and weren't going to be competing for championships as a core into their late 30s. I truly believe that when he talked to management, the deal was "we'll pay you well, you get a couple more shots with this team, but the back-end of your contract is going to be providing continuity and stability as we transition to the next phase". Doc agreed but as soon as that transition happened, he noped out and wanted to just compete for championships with veteran teams (hence going to the fully formed Clippers and 76ers). Which, fine, you don't like working with rookies and re-builds, leave that to people at the end of their career with few options or up-and-comers trying to make a name for themselves, I appreciate that. But don't sign a contract knowing that's part of your assignment and then just ditch when people ask you to actually honor that. Compounding that, he insisted that wasn't the reason he left but I still don't even understand the reason he gave.
2. I haven't followed him closely since, but it used to bother me when he coached the Celtics that every loss was the player's fault. "We got outhustled", "We didn't show up", "The guys need to play better and they know that", "No way you can win when the other team comes out and competes and we take plays off" were all constant refrains after a loss. Maybe it was true, but it seemed like he was constantly throwing his own team under the bus and couldn't even muster the "we got outcoached" or "I need to make the adjustments" or "you can't put this on any one person" nonsense that coaches usually spit out to at least cover their guys.
3. He is not a bad coach, but he has to be one of the most over-rated coaches. The last 15 years, he has only coached teams with championship aspirations and they haven't gone that deep in the playoffs. There are a whole bunch of articles out today about his teams not being able to hold leads and him losing something like 10 game 7s in his career and no other coach has ever lost more than 5.