I don't like Johnny Manziel as a player.
He has some fixable issues but a lot of my issues with him cannot or probably just will not be fixed.
I don't like his size. I don't think height is as important as most people do but it is still an issue. He is too reliant on his legs. He got better at this in his second year but his legs are still his primary weapon instead of his arm. He relies far too much on improvisation and doesn't stay on schedule. He won't be able to out-athlete everybody at the next level and will need to learn how to stay within the concept of the designed play instead of turning three out of every five plays into backyard football.
He throws off of his back foot far too often. He consistently makes dangerous throws into coverage or down the middle of the field because he knows his WRs are bigger and stronger than the defenders, something he may not have in the NFL.
He plays in a glorified high school offense with basically zero mental responsibility from the QB. All the calls, protections, and adjustments are called from the sideline. Nothing about his offense cleanly translates to the NFL, and nothing in his offense has prepared him for the NFL. You will be starting from scratch. I do believe that he has the mental capacity to learn a NFL system, but it is still an issue.
I think that his style of play will need to be adapted to the NFL, thus you may not be getting the same prospect he was in college. I believe his current style of play will lead to a few good/great years at best but not long term success. I'm not comfortable with spending the #1 overall pick on a guy who's style of play does not lend itself to long term success. You could alter his style of play. But I'm not comfortable spending the #1 pick on a prospect that you have to fundamentally alter.
For me, Manziel's absolute best case scenario is Jeff Garcia. With all the things standing in the way of him becoming that, I'm not comfortable taking him #1 overall.