I don't think much of what you said is true, yet, of Crowder himself.
Plus, Amir already had a pretty big role in Toronto.
He got a fair amount of minutes, but his USG% was quite low, lower than Sully's, KO's, and Zeller's.
If I had to guess, part of the reason he decided so quickly to play in Boston (even with his second year non guaranteed) was that they told him he could shoot more. I expect his usage to go up in Boston, mainly because he is going to take more 3's than he ever has in his career.
His scoring isn't going to be what people love about him though. Johnson sets some of the best screens in the game, making contact on the screen every time. He is going to free teammates for wide open looks and will eventually free himself for easy rolls to the hoop.
On defense he is in the right place 99% of the time and has the length/athleticism to challenge shots at the rim.
I think he's the next Crowder in a couple ways. Johnson is a player who always plays hard and will fit seamlessly into the Celtics culture (just like Crowder). He is a player that will see more offensive freedom than at any point in his career (like Crowder)