I think the Rondo experience left a bitter taste in many people's mouths. So that plays a part in it.
I also think that his intelligence is seen as a negative because of the way he speaks of Boston. The fact that he brings up Harvard and MIT seem to portray this kid as someone who is more excited about education than basketball. Couple that with the fact that he is a very, very well spoken individual, who clearly seems to think his thoughts out before he says then, and suddenly you have this perception that he's too smart for the NBA.
It's also important to understand where his true intentions lie. He's a well spoken young man, but I recall hearing the word education, or a reference to it, many times in the past few days. I heard him talk about culture, about life, about educational opportunities in Boston. He still spoke about basketball a ton, but I (personally) didn't get the same vibe of interest in basketball from him that I got when he was talking about Bostons other facets.
About being able to strike a connection with teammates. Now, the following is by no means a way for me to toot my own horn, show off anything, etc: but I consider myself to be a fairly intelligent individual. I won't go into any more specifics than that, but, anyways: I'm sitting on my couch right now; if Jaylen brown were sitting across from me, I feel (and I might be wrong about this), but I feel that I could be able to make a connection with Jaylen rather easily. And I think Jaylen would likewise be able to make a connection with me. But if an Evan Turner, or a Kelly Olynyk, was sitting across from me, I'm not sure that I could be able to make a connection with them. I'm sure Evan would try to make a connection with me, but given his personality, I just don't see it happening. Don't get me wrong, I love Evan as a person and he seems to be (by all accounts) a very funny dude, great guy, but I'm not sure I (or Jaylen brown) could be able to connect with him.
Finally (and this is the toughest to explain), he just strikes me as being different. I don't know what it is about him, whether it's the way he talks, the always apparent formal attire, the hair (lol), I don't know what it is about him, but it just strikes me differently. And I don't wanna say "it strikes me off", because that's not true, although it may seem that way from what I say (and I know I said earlier I feel I could connect with him rather easily), but it's just something about him that gives me a little bit of hesitation that basketball might not be his primary focus. Which is still ok, just as long as he realizes that his future in the NBA can be much better than his future as a professor, or a PhD (no disrespect to those with this degree, by the way; awesome job!)
I don't know, maybe it's just the Rondo experience still in my head. Maybe I'm just overreacting.