Despite all of the good things he brings to the game, he cannot be the No. 1 player on a team if they want to win a championship.
There are superstars in the league who have the ability to put the team on their back and lead them to victory. In addition, they always seem to come through in the clutch and the best of the best are able to lead their team to a championship.
Iguodala does not give those capabilities like a No. 1 player does.
But keep in mind this is also where fans give him unfair criticism. They are expecting him to be a No. 1 player when that is not what he is meant to be. Instead, Iguodala is meant to be a complementary player to a superstar (looking around... no superstar. This is awkward.)
The thing is, he just so happens to be the most talented player with a pedigree on the team and is forced to be the No. 1 option. He cannot control the situation either. It's quite the unfortunate scenario.
Even though it should be understood that it is not his fault, the fact of the matter is he is still the No. 1 guy on the team. When he is the No. 1 option, they cannot win.