Why is Gandalf considered a great wizard?
Not to knock the guy, but he really doesn't do much in the way of spellcasting. He's got a nifty sword, had a staff until he broke it, has a lesser ring...but not a lot of firepower. Elminster would smoke him with a single "Magic Missle" spell. In WoW, my level 73 gnome mage farts for more DPS (damage per second) than Gandalf has ever done. In the original Everquest, the infamous Oakbrow Farwalker could have quadkited Balrogs and Ring Wraiths without breaking a sweat. (that version of Oakie was known to solo old world dragons and other outlandish stunts--it was good to be a druid then). Hell, even Giles from the Buffyverse has shown more magical ability than Gandalf (granted with some channeling power from other Watchers or somesuch).
Seems like I almost remember an article in Dragon magazine or somewhere claiming Gandalf was about a 5th level magic user under AD&D rules. 5th level isn't particularly powerful--1 3rd level spell, 2 second level spells, 3 first level spells not counting inteligence bonuses if any.