The first books (Dragonflight, Dragonquest, The White Dragon) were quite good. They held up even on later re-reads. The Harper Hall books were also decent, though aimed at a different audience. I thought the latter added greatly to the reality of Pern — considering we see mostly the “nobility” in the first books — since these were far more focused on the underclasses; albeit mostly a privileged section of the underclass along with some details about drudges, engineers, and farmers.
I don’t recall her shying away from the sexuality. That was meant to be a feature, as far as I can tell. Remember that at the time she was competing with fantasy like the Gor and Conan stories, with burly manly men raping their way across continents, enslaving the prettiest women, and generally being males behaving quite badly.
Her books, on the other hand, had women in charge of the weyr, with the men being partly incidental to the power structure. She’s quite explicit about Lessa’s ambition and manipulativeness. Lessa exploits her own sexuality and the promise of power the rider of the bronze (or brown) who successfully mates with the queen gold will gain. There are major female characters who aren’t stupid simpering girls, which was probably pretty damn progressive at the time.
She also mentions several times that the orgies from mating flights lead to indiscriminate fucking: male-male, male-female, female-female, big pile o’ humping bodies, whatevs. Again, in the late-60s early-70s mostly adult-oriented market, this was a selling point. The dragon mating = whole lot of fucking thing didn’t get toned down until at least the Harper Hall series, and if I remember right Menolly had some serious bleedover from her fire lizards when they mated.
Thread itself is an environmental menace. It’s not evil, it just is. It’s mindless, but dangerous. There’s no magic, unless you dismiss the very circumscribed psionics as magic.
Aside from threadfall, there were other obvious SF elements built into Pern from the beginning. You start out thinking it’s a standard Fantasy world, but early on she introduces the eyestones, which are obviously stone orbital-mechanics calculators like Stonehenge probably was (at least in part). Then there’s calculating threadfall using the log tables of the ancients, which reads like an application of the gravitational influence of Pern, the Red Star, and Pern’s moons to figure out where the stuff will be de-orbiting. She has them re-engineering flame throwers, optics, and metallurgy, and introduces scraps of writings that refer to spaceships and fliers. She even has linguistic twists to present futuristic elements with the feeling that they occurred very far in the past for these people, after their technology and society fell to a level this world could support.
I think it’s actually semi-hard SF that she explores the idea of going between so thoroughly. If you can travel in space instantaneously, then of course you can travel in time. Frankly, any story that posits warp/jump/hyper drive to solve space travel and doesn’t also at least offer a limp-wristed wave to distract you from the other end of the Lorentz equations is counting on you to either be ignorant of the space-time connection, or to not give a flying rodent’s ass about it. She makes sure that time travel isn’t without cost and so does have very definite limitations, which keeps it from being too much of a plot crutch.
She also deals right from the beginning with logistics and economics. Benden Weyr is right on the edge of a peasant rebellion when threadfall happens again, because the interregnum has been so long that nearly everyone thinks it won’t happen again. Even the dragonriders more than half believe it, so many of them have become decadent and exploitive. Having enough supplies, riders, and dragons and getting everything and everyone to and from places is shown as a constant challenge even with the dragons having instantaneous travel and almost unlimited lifting power.
The later 80s and 90s books do dissolve into My Pretty Dragon silliness eventually, but the first 3, and to some extent the Harper books, I think are better than decent SF with a strong F flavor.