On paper, Brown was the one that was most likely to fill a need. The team needed another player on the wing since Turner's departure was pretty inevitable, and they needed someone who could contribute right away. Brown was the only wing and he had an NBA-ready body.
Sure the team needed a big, but the bigs that were going to be available (Bender, Chriss) were deemed "projects", which this team didn't want to take on. This draft had guards who I think are going to be good (Dunn, Hield, and Murray) but that would have just added to the surplus of guards the team already had.
I think fans were angry at the Brown pick primarily because it meant the Celtics weren't getting Jimmy Butler. If no rumors of a potential Jimmy Butler trade had been announced earlier, we all probably would have been okay with the pick.