Of the three lines spun off from the original Masters of the Universe, only the Princess of Power series had any real success. The Powers of Grayskull saga was cancelled before it even got off the ground, and while the New Adventures of He-Man made it as long as PoP did as a toyline, it only lasted one season as a cartoon and is now hardly even remembered by most.
The stories of She-Ra, though, gathered a solid following. While there's a substantial degree of overlap between their fan base and that of He-Man, I also think that there's a bit of difference between the two.
Part of that difference is gender-based, of course -- Princess of Power was designed to appeal more to girls, and did so quite well, while Masters of the Universe captured a strong following among boys. But part of that is, despite their origins and frequent crossovers, the two series differ in many ways.
I admit that for me, an analysis of She-Ra is in many ways an outsider looking in. I've always considered myself primarily a fan of He-Man and the Masters of the Universe, although I enjoyed and appreciate PoP. Some of my analysis here will be drawn from episode analysis on the masterful Episode Review Website, run by Busta Toons and Zadoc Angell; thanks go out to both of them.
The saga of She-Ra and Etheria was originally hyped as a He-Man spinoff. The standard introduction to the series starts off with Adora introducing herself as "He-Man's twin sister", and He-Man got top billing on both of the major crossovers between the two series. Indeed, the first place I heard about the movie was in the Summer 1985 issue of the He-Man and the Masters of the Universe Magazine.
In the cartoon, that element never went away completely. Not only did He-Man, Skeletor, Orko, and other MotU favorites make numerous appearances on the cartoon and in some other media, but as most people know, the elements of the She-Ra, Princess of Power cartoon are actually a combination of Mattel's original line and the Horde sub-line from Masters of the Universe.
The toy line, however, pretty much dumped any He-Man connection after the first mini-comic was released. Indeed, I would argue that in many ways, the toy line became 'Barbie with magic powers', especially during its second year. The characters became more cute than heroic, and the storylines became tales of Catra's petty jealousy towards Adora and her scheming to win Bow's heart. This divergence from the epic adventure of Masters of the Universe probably further alienated boys who would already have been reluctant to buy a 'girl's toy'. As for the young girls, I wonder if the toy line was damaged by trying to please too many and not pleasing anyone. It wasn't adventurous enough for those girls who admired Teela and Evil-Lyn, not cute enough to compete with the Care Bears and similar lines that emphasized the cute and cuddly, and it didn't have the cultural roots to hold up against Barbie.
The She-Ra, Princess of Power cartoon, on the other hand, did gather a fair following both among He-Man fans and among others. Part of this is because of the incorporation of Masters of the Universe elements, but a larger factor, I think, had to do with the fact that the story and characters were more compelling than those of the mini-comics.
In many ways, the tale of the Horde and the Great Rebellion is the flip side of the struggle between He-Man and Skeletor. While the Hero of Eternia is fighting against an invader who seeks to conquer his world, on Etheria, the invaders won years ago, and now the struggle is to drive them from the world and restore freedom.
Thus, the story's a lot easier to take seriously as a whole than the mini-comics were. In addition, the characters were more strongly drawn, although I think that has as much to do with the possibilities of a TV series as opposed to mini-comics and the comparative skills of the writers.
There were other ways in which She-Ra diverged from He-Man. There was a stronger romantic component, with the Adora/Sea Hawk, She-Ra/Bow, and Bow/Glimmer attractions, as well as other one-shot romances. In addition, while both sides on Eternia made extensive use of magic and technology, there was a strong split between the two on Etheria, as the magic-wielding heroines of the Rebellion struggled against the mechanized might of the Horde armies.
The Horde/Rebellion conflict had its strengths as well. In many episodes, we did get a feeling of the oppressive regime the Rebels were fighting against. The Horde may have lost a sense of competence as the Troopers became buffoons and walking into the Fright Zone became too easy, but we never doubted that Horde Prime and his minions were trying to squeeze Etheria dry, and that the Rebellion was a noble cause worth fighting for.
Yet despite all this, did the show really establish its own identity? I'm not sure it did. Part of that's because I am, as I said, a He-Man fan first and foremost, and thus may have problems appreciating She-Ra for its own sake. Part of it may be the difficulty in getting lightning to strike twice. But I think a lot of it was that in all honesty, while Eternia and its denizens developed a sense of depth and reality, Etheria never reached that point.
A lot of the problem lay in the characters. I admit bias here, but She-Ra's characters seem to have neither the color nor the depth that He-Man's did. Adora/She-Ra had few of the internal or external problems that Adam/He-Man was challenged with. While her fight was objectively harder, only a handful of episodes made it seem that way, and she rarely faced the challenges of identity that were so much a part of He-Man's double life. Indeed, I've long thought that there was little or no real reason for the secret identity aspect of She-Ra beyond trying to repeat He-Man's success.
The rest of the Great Rebellion fared little better. Glimmer and Bow, the two most commonly used Rebels, were too often minor help at best, victims for the Horde at worst. The other Rebels had their moments of glory, but none of them rose to the levels of Man-at- Arms, Teela, or the Sorceress. And Loo-Kee, in my opinion, is in the running for most annoying character in the Masters of the Universe mythos, up there with Caz (of New Adventures infamy) or the Starchild.
The Evil Horde, meanwhile, suffered some of the same problems as Skeletor and the Evil Warriors did in the original series, but even moreso. Shadow Weaver, like Evil-Lyn, always managed to maintain her mystique, but the other villains didn't get treated nearly so well. Mantenna became even more comic and pathetic than Beast Man at his worst, and while Catra and Modulok had solid personalities, none of the other villains even reached the levels of Mer-Man, Trap Jaw, Tri-Klops, and Skeletor's other second-stringers.
As for Hordak... he started out with so much promise and potential, as an evil warlord who actually had conquered a planet, but now had to face a rebellion that was more successful than it had any right to be. Instead, after the Secret of the Sword wound up, Hordak became much more humorous and bombastic, instead of cold and calculating. This still could have worked, but Hordak was also made into a buffoon in too many episodes. (The presence of Imp as his best friend didn't help any in this regard, I would argue.)
I realize that much of this sounds like a negative comparison between the two shows, as if I'm condemning Princess of Power for not being more like Masters of the Universe. However, as She-Ra is a spinoff of He-Man, I think it needs to be considered in that light. And in all honesty, while She-Ra, Princess of Powers has many worthy elements, I don't think it ever transcended its origins to stand as an equal to He-Man and the Masters of the Universe.