I love Ibra, mostly because he plays for my favourite team and he is one of the most entertaining players I've ever seen. He has one huge problem though: he horribly melts down in big games.
That's for two reasons:
1.He can't handle the pressure. Because of his amazing talents and his temper and/or charisma wherever he's been he always was the guy under the spotlight. Think of LeBron, if his team loses it's him who everytime takes the most of the heat by fans and press. Some players can handle it, some cannot. Ibra is in the latter category.
2.Playing style. The biggest knock on Zlatan's career is that he's never won a Champion's League. On top of that two of his former teams (Inter and Barça) won it immediately after he left. Ibra completely polarizes the offense, to make the most of his talents you have to completely build your offensive system around him. The problem is that he is a ballhog and slows down the game big time. Playing slow, giving the ball to Zlatan with his back to the goal and make your midfielders and forwards run into the spaces works extremely well against mid to low tier teams in your national championship, not so well against better, more organized teams.
Unless you're Barcelona in order to win against very good teams you need to play fast and break down the opposing defensive line, that's why Kakà won the Champion's League basically all by himself in 2007 and Ibra will probably never win it.
Ibrahimovic's talent is not in question, I've never seen a player that big move so well and to the things he does. Talent wise he is one of the best 3-4 pure forwards in the world. He is overrated as a football player though, because he doesn't play a truly winning brand of football.