The 1970's itself was a fantastic period for the genres, giving us many authors and stories.
The old guard, the writers who were writing during the pulp magazine era, were on their last hurrah. Their final few books were published, usually collections of their short fiction.
Following were the writers of the pulp novel era, who wrote everything and wrote it fast. Their job was to fill up the book racks with titles. Media writers cranked out adaptations of TV shows or films in multiple genres, not just SF. With no video market, a primary way of experiencing reruns was to read the books.
New writers were the ones breaking open the gates of fantasy. With Tolkien's success came a predictable expansion of the genre. Their new style of fantasy would sell like gangbusters, dominating fantasy from the 1980's until the 2000’s. In the early 70’s, things were just getting started.
Finally, there were also young adult and children’s writers, producing all sorts of age-appropriate fantasy. Although some have now been recognized as fantasy writers, many remain unrecognized as they didn't write the "right" sort of fantasy.
Genres were categorized differently back then. Fantasy stories were fantasy, SF stories were science fiction, and anything that fell in between was also science fiction, no matter how implausible or fantastical. These days (2000's+), anything that falls in between usually gets categorized as fantasy.
At the beginning of the decade, publishers mostly issued stand-alone books, just perfect for impulse buys, but by the end of the decade, and going into the 80’s, the series became more profitable, with trilogies becoming the norm.
If there's any theme behind the publication of fantasy, it was throwing spaghetti at the wall and seeing what stuck. As an emerging market, nobody knew what really sold, where the genre would go, or even what the genre would look like. All sorts of small publishers took all kinds of risks on all sorts of writers, putting out books that read like genre mashups by today's standards. In my opinion, that's what makes the era so fun. By the early 80's, larger publishers had begun buying up smaller publishers, and decisions for what got published slowly left the hands of the editors. Focus shifted away from catching the next big wave and focused more on producing predictable profits. Sell to the largest market, which turned out to be single white males. By the late 80's, the already biased industry had double-down on its bias
With fantasy being such an open genre in the 70's, many of today's tropes and genres hallmarks didn't yet appear in their modern forms, and in many cases, didn't exist yet at all. The new game D&D had yet to stamp its feel upon the genre. The big D&D boom, which had begun in the late 70's, wouldn't reach its full potential until the mid-80', when TSR issued its own novel line. In the 70's, most writers hadn't heard of the game, let alone played it, so roles and themes which we now consider iconic in the genre hadn't yet come to fruition, even if they did exist in the literature. Dwarves could easily be gnome-like creatures, and elves could easily be little guys with curly-toed shoes. Vampires still wore tuxedos.
Meanwhile, SF continue putting out stand-alone novels at a fast pace. Where a novel was successful, they would publish more, creating ad-hoc series or stand-alones in the same universe. The idea of trilogies came later to SF than fantasy, with many authors producing stand-alone novels well into the 80's. Many 70's SF novels were speculative to the extreme, based on a "what if" idea that would be utterly impossible under any science, yet still played out against reality. At the same time, hard SF would see its own blooming, producing the likes of Niven and Pournell.