Jaylen Brown is a really hard guy to read, in terms of potential.
On one hand, he has god good size, great strength and great athleticism for the Small Forward spot along with the physical tools to be able to play as a small ball PF in some lineups.
On the other hand, he doesn't look like much more then a role play or a 5th / 6th man to me in his current state. Doesn't seem to be a great scorer, great ball handler, great passer or great shooter - and I'm not sure if he has the ability to become either of those things.
But then if you look at guys with similar game and physicals to Brown from the past (Winslow, Stanley Johnson, Jae Crowder, Kawhi Leonard, Kenneth Faried, Butler, Ariza, etc) pretty much all of those guys have become contributers at the NBA level. That means the actual floor on Brown is probably reasonably high - it's hard to see him not becoming at least a solid starter, and history seems to suggest he has a fairly good chance at becoming a fringe star.
Ultimately I think that Brown is a good pick if you're selecting anywhere in the #5 - #8 range, but I feel he's a major reach with anything in the #1 - #4 range.