Off-the-field stuff aside, I've just never really been a fan of his game in terms of translating to the NFL from what we've seen SO FAR. Sure he can improvise and make plays on the move but he hasn't shown enough as a pocket passer yet. And yes, even though there is a wave of read-option QBs in the NFL now, let's compare him to the successful ones so far like RG3, Russell Wilson, and Kaepernick. In the case of the first two QBs, they showed ability to be a pocket passer in college in addition to their running skills. In Kaepernick's case, he isn't as good of a passer yet (he can still zip the ball though) but he's much bigger than Manziel.
Gotta see what Manziel does this season in terms of developing his game but I'm sure it will be hard with all the media distractions and the target on his back now.