I was in the tub a few days ago--I, like most people, do a lot of good thinking in the bathroom--and came upon a bit of a revelation.
I've been gaming for just about 15 years now. While I started in the midst of Dungeons and Dragons' second edition, I was quite familiar with the first edition rules, as well as that of Palladium's various systems.
The very foundation of the Dungeons and Dragons concept of magic revolves around a bastardized version of the magic system used by author Jack Vance, in his Dying Earth series. Within gaming circles, this knowledge is not uncommon. Vance was one of the first popular fantasy writers to come up with the idea of magic as a 'fire and forget' system. Your first level wizard would prepare/memorize/ready his magic missile, but when that spell was cast, it was gone for at least 8 hours.
This concept of 'fire and forget' magic translated easily into D&D, splitting evenly along the lines of divine and arcane magics. It was simple for a player to tell how many spells he had left, what level of spells were available, and what he could do at any given time. That said, many gamers felt frustrated with their inability to replicate the iconic spellcasters of fantasy and folklore.
And, seriously...who can blame them? Tolkien didn't exactly sit around worrying how many 8th level spells Gandalf can pump out in a day, or whether Saruman memorized that Telekinesis spell with a metamagic effect on it. Robert E. Howard didn't spend time figuring out whether Thulsa-Doom should use Polymorph Self or Shapechange. Tennyson and Malory didn't exactly worry so much about whether Merlin had cantrips or not.
This debate went on into third edition and its 'successor', 3.5e. While revolutionary, particularly in terms of the System Reference Document and the Open Gaming license, the Vancian-based magic system continued in terms of both wizard and sorcerer, both druid and cleric, and even the lowly bard. Always, we seemed pinned into 'fire and forget'.
This is not to say that other systems have not gone in other directions. Countless systems have. Even D&D has, to lesser degrees, through its psionics system, even in second edition. The problem there, though, is the fact that psionics is a marginalized segment, even within the more open scope of third edition. Within second, the system was stigmatized for balance issues, making playability and popularity minimal.
However, after the advent of 3.5e, Complete Arcane was released. And with CArc came a novel concept--the Warlock. For the first time in the history of Dungeons and Dragons, players were given a spellcaster who was not limited by spell slots or memorization. Instead, it was merely a matter of knowing or not knowing an invocation.
The uproar, at least on the Wizards of the Coast message boards, was immense. Many players simply did not comprehend how to play the class. Where a wizard or sorcerer would simply run out of spells, the warlock could run all day, non-stop, yet still remain balanced with its Vancian counterparts.
From there, the warlock's popularity led to further innovation within the D&D magic system. Tome of Magic, featuring the TrueNamer and the Binder, both featured functional magic systems that were comparable to those of the wizard and sorcerer, yet provided unique alternatives. The binder had access to any of its abilities, once per five rounds. The truenamer could use its highest abilities over and over again, provided he could make his ever-escalating true-name check.
Magic of Incarnum pushed the envelope farther, impinging on the realms of New Age mysticism and Far Eastern metaphysics through chakra binds and soul melds. Again, these did not require spell slots or memorization--just pick what you like. The higher you get in level, the more earth-shaking things you can do.
So different, so innovative were these concepts within D&D that they have spread out of the realm of magic and into the realm of melee combat, through Tome of Battle: The Book of Nine Swords. Again, rather than readying maneuvers based on a Vancian system of x per day, we find abilities that can be refreshed in mid-combat, and are automatically regained following a battle.
Now, what does this mean for the discerning player? Well, by and large, it means more fun. Players enjoy seeing their characters at their best. When we're low on hit points, low on spells, low on resources, it becomes difficult to succeed at any major task. The priority for the player and for the character becomes resting, not adventuring. We, as players, play the game to fictitiously adventure, not to fictiously rest. These new systems accentuate this style of play, moving on the action faster and more furiously, pleasing both players and DMs.
What does this mean for the industry? This, above all else, shows the gilded age of D&D that has been upon us. For 25 years, authors have felt constrained to comply with the Vancian system, simply for verisimilitude. For the first time, we have authors, on a large scale, stretching their wings within the D&D engine. New systems, new concepts, mean more innovation across the board.
Needless to say, this sort of change is hard to fathom, coming from only one base class in a splatbook. But, the Warlock, for the first time, provided a divergent point of view--showing us that D&D can still be D&D without spell slots...and sometimes can even be more fun for it. The Warlock and its ilk will never replace with typical Vancian wizards and sorcerers of D&D--there's too much heritage there to fully overcome that massive mountain--but, the point remains that D&D has changed...particularly, changed for the better.