Release of word/sound-stored energy is not particularly debilitating to the spell caster, as he or she has gathered this energy over a course of time prior to the loosing of the power. It comes from outside the spell caster, not from his or her own vital essence...[B]ecause spells tap power from other planes, any improper casting is likely to cause the spell not to function... (Gary Gygax, Dungeon Master's Guide, 1st ed., p.40)
The tomes which held Turjan's sorcery lay on the long table of black steel or were thrust helter-skelter into shelves. These were volumes compiled by many wizards of the past, untidy folios collected by the Sage, leather-bound librams setting forth the syllables of a hundred powerful spells, so cogent that Turjan's brain could know but four at a time. (Jack Vance, "Turjan of Miir", from The Dying Earth)
Magic is not an exact science in Gygaxian D&D. Magic-Users study, learn their spells, absorb the words of power and marshall their material components, then cast them; I think that term is appropriate, not least because it summons up an image of a quarterback throwing a Hail Mary pass. The power comes from other planes, other dimensions, and no matter how skilled a mortal becomes in its use, there's still the danger of failure and unintended side effects.
I like that. It makes magic eerie and wondrous, no matter how familiar I become with the game. Something in your brain disappears as soon as you use it, as though the spell were some kind of symbiotic entity waiting to be unleashed. How freaky is that?
I also acknowledge that the Vancian explanation of magic neatly limits the power of Magic-Users, especially at lower levels, and I have no doubt that it was this game consideration that led Gygax and Arneson to adopt the "fire and forget" aspect of the Dying Earth's magic. But they could have done otherwise. They could have made spells less potent; they could have had made magic an alien technology that needed periodic recharging; they could have decided that expensive and hard-to-come-by material components were necessary for all spells. But of all the limits they could have placed on magic, they selected this one. That D&D uses Fire and Forget is a sign that the weirdness of Vancian magic was intended to be a part of the feel of D&D, and not just a mechanical limit on the class. (The other sign is the inspiration for this post's title: "Vecna" is an anagram of "Vance".)
For this reason, I'm not fond of Sorcerors in Pathfinder, or feats such as Eschew Components. Nor am I a fan of insisting on using the term "prepare" instead of "memorize" for spells in 3.5. And the "At-Will" and "Per Encounter" powers of 4th edition completely break with Vancian tradition. Each of these is an attempt to change the specific flavor that the game's creators deliberately put there. I said it to a friend a little while back: it's cool that you like a game that doesn't have Vancian magic; you just like a game that isn't D&D.
I know that sounds a little smug, but I really mean it about the cool part. Lots of great games aren't D&D, and I've had fun playing some of them. I even had a good time playing one game of 4th Edition. And I think there's plenty of room to improve the mechanics of the game in ways that weren't obvious in 1974. But for me, it's only a homonym for the game I love unless it sticks to a few main ingredients, and one of those is the Vancian magic system.