You are correct that, overall, the classic anime has more non-plot advancing material (i.e. filler) than the manga does. On the other hand, in many cases, the anime boasts more plot and depth of storytelling, whereas the manga plows through each story arc in a more bare-bones, brief, and (arguably) flat style. As a couple of examples: 1) whether villain or ally, many characters in the manga are introduced and then promptly killed off in order to move on to the next character. Only in the anime are enduringly popular minions such as the Witches 5, Amazon Trio, and Animamates developed into personable and sympathetic characters. 2) In the manga, it's not as clear as to how the Starlights enrolling as students at Juuban High School is very necessary to advancing the plot, whereas in the anime, as a result of their schoolmates friendship with Usagi, Ami, (Rei: actually, she attends a different school but she usually comes over to their campus to hang out), Makoto, and Minako, which took the span of a whole season to develop to the point of mutual trust, leads the Starlights to end up being the only ones who stand with Sailor Moon and protect her until the end; without their encouraging pep talk, she would have given up her star seed to Sailor Galaxia in episode 198 (if she had done so in order to escape her grief, everyone in the galaxy would die permanently, the end).
(While it is the case that Sailor Moon Crystal has, for the most part, avoided filler, the storytelling is arguably lower in quality than that of the classic anime [Crystal's viewer numbers have sharply crashed, a drop of 77%]).
The number of changes in the classic anime from the original manga is so high that it would take an online encyclopedia to list them all. As just one narrowed example, consider the LGBT content:
The classic anime invented the following popular LGBT-related content which was NOT present in Takeuchi’s original manga (and thus, is NOT present in the reboot):
~ Kunzite/Zoicite: in contrast to serving as the 1991 anime’s sole canon gay couple, in the original manga, Kunzite had a straight romantic relationship with Sailor Venus (Codename ha Sailor V volume 3), and Zoicite is shown in a manga illustration with his arms around Mercury, implying that they were also a heterosexual couple on the Moon Kingdom. The live-action Sera Myu (Sailor Moon musicals) incarnations follow these pairings. The 2013 La Reconquista musical paired up the male generals with each of the female sailor soldier guardians at a ball, and particularly highlights a prominent and cute relationship between Zoicite and Mercury. The completed 2003-2004 Pretty Guardian Sailor Moon live-action TV series does not include any romances for Kunzite or Zoicite.
~ Fiore: one of the classic anime’s original characters, featured in the R movie and the Another Story video game, could be interpreted as liking Mamoru, though this was never explicitly affirmed or denied.
~ Fish Eye: a gay cross-dresser who fell for Mamoru in the classic anime, is, in the manga, merely an mildly-effeminate monster-of-the-day (the same could be said of Hawk’s Eye and Tiger’s Eye - all three of their outfits are… unique). In the manga, he never cross-dresses or expresses any homosexual interest. He only attempts to seduce Ami (Mercury).
~ Sailor Starlights: in the classic anime, this trio flipped back and forth between female and male when they transformed from sailor soldiers into civilian guise and developed intriguing relationships to varying degrees with the Earthling sailor soldiers but, in the manga, there are no fun Taiki (Star Maker)/Ami (Mercury), Taiki/Makoto (Jupiter), Yaten (Star Healer)/Minako (Venus), or Yaten /Luna vibes. Those were introduced in the anime along with the sex-switching that challenged gender norms. In the manga original, Ami hates Taiki, Michiru (Neptune) hates Yaten, and Haruka (Uranus) hates all three - that’s really the entire extent of the interaction between these extraterrestrials and the solar system soldiers. Usagi, Minako, and Makoto blush and get heart eyes when the Three Lights transfer to their high school, that’s it. Even Yaten’s passionate and undistracted devotion to Kakyuu Princess is original to the classic anime, and the anime’s Haruka vs. Seiya rivalry and Seiya/Michiru flirting were not in the original manga… heck, basically anything you liked about the Starlights in the classic anime wouldn’t show up in a reboot. Taiki’s softening toward his astronomy teacher and his sweet hospitalized fan or laughing for the first time in years; Yaten as a cat person who constantly flips his bangs in annoyance; Seiya’s boy-next-door softball training, amusement park date, and conflicted, unrequited love in the vein of Taylor Swift’s You Belong With Me that spanned 28 episodes (82% of the final season) with Usagi? That’s all original to the classic anime.
The manga Three Lights are always female and demonstrate zero personal preference for passing as male. Manga Yaten and Taiki are merely girls without any sexual preferences indicated, who disguise themselves as boys for a mission. Since neither of these characters display any attraction to anyone, they’re not lesbians. Since they’re not particularly masculine by any stretch of the imagination, don’t identify as male, don’t evidence any interest in gender identity whatsoever, and wear feminine clothing in private, even attempting to identify them with cross-dressers is a little shaky - in their minds, their boy band costuming is just a disguise rather than a conscious lifestyle choice.
That leaves us with Seiya (Star Fighter) alone, who is attracted to Usagi in the manga.
Seiya (Star Fighter)/Kakyuu: it can be interpreted that manga Star Fighter had feelings for Kakyuu Princess before arriving on Earth but this is not overt. Kakyuu resting her head on Seiya’s shoulder was only shown in the classic anime, not in the manga.
In contrast, in the original manga, there was:
~ Sex-switching Usagi (Moon) and Minako (Venus), which was not included in the anime. Many English-language websites bemoan the fact that Takeuchi-sensei was shocked that, in the classic anime, the Starlights were male in civilian form - but she’s the one who thought up sex-switching in the first place. In the manga, Usagi uses the Hensou Pen (Disguise Pen) to transform herself into a man (a groom [pages 190-191 of volume 1 of the Kanzenban {Perfect Edition}]) and Minako uses her compact to transform into a male in BSSM itself (a Mugen Gakuen [Infinity Academy] student [pages 167-169 of volume 5 of the Kanzenban]) using the phrase “Moon Power!” (「ムーン・パワー」). As an aside, Minako also twice used her compact to transform into male form in Codename ha Sailor V (a teen idol [pages 115-118 of volume 1 of the Shinsoubon {Reprint Edition}] and a samurai [pages 140-141 of volume 2 of the Shinsoubon]), both using the phrase “Mikazuki Power Transform!” (「三日月パワー・トランスフォーム！」).
~ Haruka (Uranus)/Michiru (Neptune): yes, they have a romantic relationship in the manga, BUT very little is shown or said of it when you contrast it with their classic anime version: although you would see a good amount of Haruka/Usagi action including their manga kiss, manga Haruka/Michiru romantic scenes and dialogue are sparse. Almost all of their suggestive lines, Haruka’s clearly stated answer to what her koibito’s name was ("koibito" is a Japanese word which can mean either simply "girlfriend"/"boyfriend" or can mean "lover"; usually in the context of shoujo anime, it refers to the former), and their moving interlude after Galaxia removed their bracelets originated in the classic anime. Fans who met Haruka through the anime might also happen to be surprised to discover that her manga version wears a high school girl’s school uniform, mini skirts, girly blouses that show off her cleavage, and a polka-dotted scarf - in the manga, sometimes she presents as a man, but other times she’s rather feminine; it was only in the older anime that she consistently dressed masculine.
~ Haruka (Uranus)/Usagi (Moon): Haruka/Michiru fans may be disappointed to realize that there’s a lot more Haruka/Usagi than Haruka/Michiru visuals in the manga, and there’s also a brief Mamoru/Michiru vibe. People who like the idea of Haruka/Michiru being monogamous would be challenged by manga Haruka’s amorous advances toward Usagi in full knowledge that Usagi has a boyfriend and while being in an apparently open relationship with Michiru. The classic anime Haruka flirted with Usagi and Minako, but it was only in 2 episodes accompanied by Michiru’s bemused comments which indicated she didn’t consider it anything serious, whereas the manga Haruka pursued Usagi a lot more seriously.
~ Seiya (Star Fighter)/Usagi (Moon): in the manga, there’s some chemistry and a kiss, but most of what Seiya does in the manga is send Usagi anonymous postcards. The duration in which the Three Lights attend school with Usagi and friends is all too brief.
So that's just one example of the countless large and small differences between the manga and the classic anime.
There are positive changes (Serenity/Sailor Moon committed suicide twice in the manga, whereas in the anime, she didn't), neutral or disputed ones (some fans like the serious manga Rei [Mars], whereas others like the anime Rei who teases Usagi as a friendly sort of love/hate rivalry), and generally-received-as-negative ones (the manga Shitennou had served Prince Endymion in the Silver Millennium and returned to loyalty, giving him strength, whereas the anime version had no backstory of their being loyal to him). The aspect where the manga/anime difference is greatest is that the end of the manga and the end of the anime are completely different. Some fans like the manga version better, some like the anime version better.
