The complement of yesterday's post questioning the feeble power of a 1st level magic-user would be to question the immense power of an 11th level magic-user. Are wizards really as powerful as we believe them to be?
In Men & Magic, an 11th level wizard has about the same hit points and combat ability as a superhero, and can cast spells of up to 5th level. How powerful is a 5th level spell? Contact Other Planes, Magic Jar, Conjure Elemental, and Teleport are the most powerful, but look at the limitations: Contact Other Planes can only be used once a week (and risks insanity,) Magic Jar requires a lot of set-up and puts the wizard at risk, Conjure Elemental requires continual concentration to avoid being killed by your own servant, and Teleport requires knowing the destination in detail, including the topography; teleporting to a location you've only seen in a crystal ball has a 75% chance of death. And remember: you only get to cast three of these per day, and you must plan ahead. For that matter, you'd better hope that you actually know those spells; depending on which interpretation of the rules you're using, you might not have any spells beyond 1st level unless you've found, purchased, or researched them yourself. Or, if using the Greyhawk Intelligence table, you might not be able to learn that Fireball spell, ever.
Wizards can get around the spell limit with magic items -- if they have spent money on a tower and laboratory. And if they have the time and money to make the item. That wizard who wants to be a master of fireballs could make a Wand of Fireballs for 10,000 gp and six months effort. Spells that the might be useful when not cast in combat could be turned into scrolls: a Pass-Wall scroll would cost 500 gp and take 5 weeks to prepare. Most magic items that a wizard found while get used up or given to someone else as payment or tribute. This explains why randomly discovered wizard towers in Underworld & Wilderness Adventures usually result in adventurers being placed under a geas to retrieve some item: wizards are too busy manufacturing magic items to go searching for others.
The thing about magic is: it requires preparation. Even when dealing with a random enemy who shows up at a wizard's tower unannounced. Sure, a Cloudkill could wipe out trespassers; but did the wizard know there were going to be trespassers? Did the wizard memorize Cloudkill this morning? Will the wind be blowing in the right direction?
What seems to happen in discussions about wizards being so powerful is that people imagine situations that the wizard just happens to have the right spells prepared for,and the opposition doesn't get the same benefit of preparation or just plain good luck. Yes, if a wizard's enemy happens to be in a location the wizard has visited and studied many times, and if the wizard has a crystal ball to confirm this, and if the wizard learned the Teleport spell, and if the wizard survives the teleport in without exploding or embedding himself in the floor, and if the wizard learned Fireball or Lightning Bolt or has a wand, and there's enough room to cast that spell, and if the enemy has no protection from those spells, and if the enemy doesn't hit the wizards with Feeblemind or Charm Person first, then a wizard can do a "scry and fry". But that's a lot of ifs, some of them rather unbelievable.