Robert Jordan's delighted letter to George R.R. Martin's editor on learning that A Song of Ice and Fire is going to be more than three volumes in length.
Epic fantasy has always been big: The Lord of the Rings, at 450,000 words, was insanely huge for its day. The Sword of Shannara, Magician and The Dragonbone Chair, all checking in at around the 300,000 word mark, were all big novels. Even smaller trilogies and series, like The Belgariad, The Black Company and The Dragonlance Chronicles, became epic when assembled in omnibus editions.
The Wheel of Time was different, though. The books were big, many clocking in at north of 300,000 words and a couple (The Shadow Rising and Lord of Chaos) falling just short of 400,000. But they were also numerous. More importantly, the whole thing was one big story. Raymond E. Feist's Riftwar Cycle and Terry Brooks's Shannara series both clocked in at 30+ books, but these consisted of stand-alone novels or sub-series with their own self-contained story arcs.
The Wheel of Time, instead, was actually one story and in fact at the 2014 Hugo Awards was allowed to be nominated as one extremely long novel. The story that begins on page 1 of The Eye of the World does not conclude until the closing page of A Memory of Light, just short of 12,000 pages later. This was something that was different, and unusual. The Wheel of Time opened the door for series that were bigger than the standard trilogies, with massive narratives allowed to sprawl unconstrained by traditional page count limitations. Authors from Kate Elliott to George R.R. Martin to Steven Erikson would benefit from this precedent. Without Jordan, the idea of a single story broken into seven or ten or fourteen massive tomes would have taken longer to become popular. Of course, some critics and readers despairing at the never-ending mega-saga in fantasy fiction may bemoan this fact as much as fans celebrate it.
The idea of a "magic system" was not new in fantasy. In fact, it predates The Lord of the Rings itself. In 1950 Jack Vance's The Dying Earth featured a system of magic based around immutable laws and learnable rituals, with wizards learning spells from books. When they cast the spell, it disappeared from their mind and had to be re-learned. These rules were so popular that Gargy Gygax later "borrowed" them for his Dungeons and Dragons roleplaying game.
Such magic systems had gone on to appear in fantasy novels, with the D&D-influenced Magician employing a system of magic divided into the "Great Path" and "Lesser Path". The Dragonlance Chronicles and R.A. Salvatore's Drizzt books, being set in the D&D multiverse, simply used the D&D magic system directly.
However, it was The Wheel of Time that popularised the idea of a magic system. James Oliver Rigney (aka Robert Jordan), quite cleverly, never once uses the word "magic" in all fourteen volumes, instead exclusively using the term "One Power". The Power is divided into male and female halves, with different ways of weaving the power and combinations of male and female channellers having different effects. The rules are complex and the characters' understanding of them constantly change throughout the books, but always remain consistent and coherent. Some fantasy authors, most famously Brandon Sanderson, would go wild developing their own magic systems. Others, like Steven Erikson and Ian Esslemont, would keep their rules carefully hidden away to encourage reader speculation on what they were like. Others still, like George R.R. Martin, would reject the notion of a magic system altogether, preferring magic to be weird, dark and mysterious like it was in Tolkien, or legend.
Obviously, The Wheel of Time did not create the idea of strong female characters in fantasy fiction: A Wizard of Earthsea, The Mists of Avalon, The Empire Trilogy and many more featured important female characters and crucial protagonists long before The Eye of the World was published. And the growing frequence of major female protagonists in modern fantasy is down more to the growing number of excellent female writers in the field as well as male writers being a bit more egalitarian than some of their forebears.
But The Wheel of Time was definitely a showcase for such changing attitudes in the 1990s. Although the main character of Rand al'Thor is male, many of the other critical characters in the series are female. The series is often represented as the story of three core male characters (Rand, Mat and Perrin), but this is actually erroneous: after Rand himself, the most important character is Egwene, and The Wheel of Time gains a new perspective if viewed as the parallel stories of Rand and Egwene and the differing paths they take to achieve their goals. Approximately two-thirds of the characters in Wheel of Time are female, many of the nations are matriarchies and for most of the length of the series, only women can use magic safely. The series is also noticeably lacking in some of the more questionable "gritty" or "realistic" misogynistic acts found in other fantasy series. The series also features fairly positive representations of lesbian and bisexual characters (although, many have wryly noted, almost no male gay characters at all until almost the very end, and then very fleetingly).
Despite all of this, the success of The Wheel of Time as a feminist - or at least feminist-nodding - work of fantasy is much more debatable (and has been debated online, at extraordinary length). Robert Jordan could use fairly broad characterisations and tended to deploy physical tics (such as tugging braids or "folding arms under breasts") fairly repetitively. Also, whilst the battle of the sexes theme is played up throughout the series, it remains at a fairly juvenile level for most of its length. It's arguably only right at the very end that the male and female characters seem to grow up and learn to respect and live alongside one another in mutual understanding. Still, the fact that the biggest fantasy series in the world adopted a very pro-women stance at a time when many other series were doubling down on lazy stereotyping or just outright sexism was in itself notable.
The Wheel of Time was supposed to be a trilogy, was contracted for six books and ended up as fourteen volumes. Unexpected expansion of the story - or, less charitably, "bloat" - is a common problem in the epic fantasy genre, with many series expanded beyond the ability of the narrative to sustain them, sometimes for creative reasons, sometimes financial.
In the case of The Wheel of Time, the author had a rough story arc mapped out in his head and the final scene written down quite early in the process, but the actual process of getting from A to B was only discovered in the writing process itself. This is the so-called "gardener" approach, also employed by the likes of J.R.R. Tolkien, George R.R. Martin and Stephen King. Like those authors, Robert Jordan encountered problems of staying on course and keeping the narrative focused. But each of these authors dealt with the problem in a different manner.
Tolkien, of course, simply completed his entire story long before publishing any of it. This allowed him to rewrite and re-edit the whole thing. This permitted him to simply chop off large sections of The Lord of the Rings which detracted from the focus of the novel. An episode in which Gandalf explains to Frodo why exactly he recruited Bilbo to take part in the events of The Hobbit was deemed unnecessary and removed to become a short story ("The Quest of Erebor", later published in Unfinished Tales). A lot of extraneous history lessons from Faramir about the history of Gondor was moved into the appendix. A lot of Aragorn and Arwen's love story likewise was relegated into the notes. All of this was done before publication, allowing the story to be refined prior to release.
Jordan, of course, couldn't do this for The Wheel of Time and this left him in a problematic state when he got to the third quarter of the series. By this point he had characters like Rand ready to pretty much head into the Last Battle, but most of the other characters were way behind. One solution, to "bench" characters who had nothing to do at the moment, proved unpopular with readers, so Jordan had to invent more material (arguably "filler") for them whilst he was bringing other characters up to speed. This led to controversial episodes like Perrin's wife being kidnapped by a minor enemy in Book 8 and not being rescued until Book 11, with Perrin's entire story arc inbetween consisting of angst. Eventually he got the series back on track and heading for a satisfactory conclusion, but even the most ardent Wheel of Time fans will generally admit to finding Books 8-10 of the series a relative struggle when compared to the rest.
George R.R. Martin's solution - to rewrite each volume multiple times during the writing process - combines the two approaches but leads to the detrimental effect of each volume taking five or more years to come out.
Newer fantasy authors seem to be aware of the bloat problem ahead of time and in some cases have taken steps to avoid it. Steven Erikson's ten-volume Malazan series avoided the issue by having a self-contained primary story in each novel and relegating the serialised elements to subplots. Brandon Sanderson has divided his 36+ novel masterplan into lots of smaller, self-contained storylines and trilogies, only obliquely referring to the serialised elements linking the whole thing together and not planning to bring these elements to greater prominence until much later.
Some fantasy authors - like a certain J.K. Rowling - also found a novel way around the problem: by coming up with a target number of books, sticking to it and executing it.
As the 1990s got underway, dominated by Robert Jordan's epic fantasy behemoth, other writers started coming to prominence and writing their own series. This was a new generation of writers emerging on both sides of the Atlantic, and further afield in Europe as well. As some of the old school also continued to produce new work, this was a time for the field to become more diverse and more interesting.