Wizards have awesome, unbridled power and nearly limitless versatility, but that power and versatility hinges on preparation and usage. If a Wizard knows, eight hours or more in advance what they will be facing and the conditions they will be facing "it" in, they are nigh unstoppable. With proper and moderate spell selection, a Wizard will defeat almost any opponent because they have a counter to almost every trick.
The problem Wizards have, and this is why I rank them slightly below Druids and perhaps Clerics, is that they suffer significantly in ambush situations, adventures where the clock is ticking, and wars of attrition. Wizards become significantly less awe inspiring in campaigns where there is more than one significant encounter per rest period. In fact, if it wasn't for the Wizards extremely versatile preparation and retreating spells, they would drop below Sorcerers in my book.
The best bet Wizards have when facing an unknown situation is Teleporting out, resting, casting Divinations and then charging into the breach. So long as they maintain that edge of preparation and foreknowledge, they will always win. So the key is to take that away from them, if you want to limit their power.
It really comes down to the type of campaign you're playing. If the villians are mostly reactive, there are rarely more than one or two encounters a day, and there's never a mission with a pressing need for a quick resolution, the Wizard is virtually a god. On the other hand, if the villians are proactive, actively harrying the PCs, there are three or more encounters during the day, and the party needs to get a great deal done in a short amount of time, the Wizard becomes what he should be, a limited usage big gun.
Until that changes, Clerics and Druids will always maintain a slight edge over Wizards, since they have as much or nearly as much awesome spellcasting power, but also other abilities to fall back on once that runs out, unlike the Wizard.