Imho, the problem with the Wizard isn't so much that he's overpowered, it's that he's the most flexible and versatile Mage - and he's strong, too!
There's nothing you cannot do with a Wizard, and you're able to have a potential answer to any kind of threat your opponent might throw against you.
What this means is that there's no matchup that is unfavorable for you, so by definition it's the best Mage to choose for a tournament.
If, however, you're in a situation where you know what to expect, i.e. what kind of Mage your opponent will play, other Mages can often be a better choice.
So, if players would prefer to see fewer Wizards in tournament, he'd either have to lose some of his flexibility (e.g. by having him play triple for some spells) or power (e.g. reduce his Channeling, maybe conditionally).