Unlike the anime, the manga arcs all flow into each other amazingly well. For example, Sailor Moon and the others get their power ups at the end of each arc, through much better means than in the anime. The manga is darker, but not without humour, and tells a much deeper and better story. Most notably is SuperS. The anime turned this into a cheesy, fluffy story, where in the manga it is very dark. Neherenia is a much more notorious villain, the amazoness quartet holds a far greater fate. Also, the golden crystal (and the silver crystal, for that matter) are much more important.
The anime also ignores the manga specials, save one. They animated "Ami Chan's first love", but not the stories focused around Jupiter, or Venus & Mars.
The main characters are much more developed individually in the manga, whereas most of their growths (save Usagi) are ignored or forgotten about in the anime. (For example, the Starlights in the anime each interact with other senshi, and have experiences that change them, but this is later forgotten about. Most notably, Ami's words to Taiki, and Minako's to Yaten)
The anime also changed many of the characters, most notably the outer senshi, Venus and Mars. Sailor Venus is a much more important character in the manga, where she was sold short in the anime. She is the leader of the inner senshi, and participates much more than in the anime. Especially in the Dark Kingdom and Black Moon arcs. Mars in the manga is a much more elegant and serious character. She and Venus are both portrayed well in the live action, but not the anime. The anime also ignores the relationship between Venus and Mars, which is very powerful, whether you regard it as romantic or not.
Sailor Saturn is also sold short in the anime. She is in S, SuperS and Stars arcs, as a major character. She interacts with the other senshi a lot more, and has a much bigger purpose. The outer senshi are much more important to the story, and never vanish like they do in the anime.
Uranus is also somewhat different, mostly in that she shows her more feminine side much more in the way she dresses. Pluto also has a bigger role, especially during the Black Moon arc.
Chibiusa is also much less annoying, mostly because you can't hear her talk, and she doesn't try and steal the spotlight so much. And, Tuxedo Kamen is actually useful! Instead of waving a cane around and throwing flowers, he actually is granted real powers by his four generals! The four generals also change a lot from the manga to the anime, and not in a good way.
The art in the manga is very elegant, and the artbooks are beautiful.
I promise that if you are a Sailor Moon anime fan, you'll love the manga. I think it is far greater, and I wish the anime had stuck to it more. The live action stayed closer, but created it's own story, but even that was better than the anime. I still appreciate the anime, but the manga is by far the better story. (I wish they would do what Fullmetal Alchemist did, and release a new version that's true to the manga!)
