Barnes defends multiple positions like Jeff Green does (i.e., not that well at any of them). He's got good shooting numbers, but has been playing in more or less the best situation possible. He's better than Green, but he's probably a below average starter in the league.
As someone who has watched (and probably rooted for) more Harrison Barnes basketball over the past three years than anyone here not in the Bay Area, I agree with this read. Barnes made a leap last season for three reasons. First, he naturally improved as he had more experience in the league.
Furthermore, the core of the Warriors jelled and he fed off of being the third or fourth option on any play when defenders sagged off him to help cover Curry, Thompson, Green and Iguodala (in the small line-up), either opening up shots for him in the corner or giving him space to get to the rim. The presence of the small line-up also forced other teams to go small and he was able to defend third/fourth option wings while Green, Iguodala and Thompson took the better players.
Lastly, Barnes usage changed significantly under Kerr to the above from running a lot of ISOs under Mark Jackson. I don't dislike Jackson the coach as much as some Warrior fans but his penchant for calling Harrison Barnes ISOs near the basket was enough to induce Smiling Joe Hesketh-like rage from even the most patient fans.
In short, Barnes is a decent player who had good numbers the past two years playing in an ideal system. If he is asked to create his own shot consistently or find points by finishing at the rim (he is not good at that) he is going to disappoint people. I cannot imagine a worse place to be a bust high priced free agent for Barnes than Philadelphia.