The 11th best manga of all time, Freezing, isn't ranked at all in anime. Why? Because the show didn't accurately represent the characters, focused on all the wrong things, and ultimately was too riddled with filler to even convey the story correctly. A lot of good manga that's heavy on nudity is mishandled by anime. They seem to be unable to cope with the pheromones floating around and lose their cool instead of focusing on the story. To Love-ru is a rare exception to that curse, but To Love-ru has its own troubles in that Xebec isn't remotely capable of retaining the incredible artistic beauty of the manga. Maybe if Kyoto Animation were willing to do To Love-ru, or UFOtable, but Xebec is just way out of its league. . .
The Mujaki no Rakuen oav's are a terrible rendition of the manga. They turn it into a big joke, and the art quality is just rock bottom. Again, the anime seems to be too embarrassed by itself to faithfully deliver what made the manga so great in the first place.
Futari Ecchi's old oav's were just as bad. However, the new 2014 oav's are actually quite brilliant. They're like Tamayura except with sex. With just three oav's though, trying to cover one of the longest manga series in the industry, there's just no way to capture the magic that is the manga. Just like the Jojo's oav's from the 90's weren't anything to write home about, but the current series that's faithfully adapting all the material is great.
I"s was also a romance story, whose foundation was the beautiful art depicting the beautiful girls, which had sex, so of course the anime version completely ruined it. A handful of oav's which make no attempt to tell the story in chronological order and have no relation in quality of art to the actual characters in the actual series, just doesn't cut it. It's clear that only manga can tastefully depict erotica because there's just too many frames in animation to do a beautiful naked girl justice. This is the primary reason every 'adult' comic ends up a failure as an anime.
Zettai Karen Children is ecchi like any shonen comic should be, because nothing's more fun for boys than pretty girls, but that's no excuse for why this manga was so horribly adapted. Even though the anime is well over 50 episodes long, it was so full of filler, random chronological displacement, and focusing on sub-characters and sub-plots that ultimately didn't matter, the whole thing was a bust. We never even got to see The Children in their middle school forms.
New Game's getting an anime this summer so one of the missing 19 is accounted for.
Full Moon O Sagashite was adapted too soon and as a result the anime was almost entirely filler. This would be a great series for a remake, ala Sailor Moon Crystal, which had the same problem.
Strobe Edge was never given an anime, though it did have a live action adaption. I think the problem here is that shoujo comics, despite having massive readership, just don't sell many blu-ray copies when turned into an anime. Women clearly aren't as attracted to the anime industry or willing to spend as much on it, so manga, which they do like and is much cheaper, can't make that jump to the screen that boy-focused shows can. Fujoshi bait like Free might sell well, but mainstream simple romance stories like Strobe Edge don't appeal to that crowd, so there's no point adapting the work at a loss.
Rosario Vampire lacked confidence in the story of the manga, and thus just tried to capitalize on the cuteness of the various monster girls and create a generic harem anime. Maybe they looked at the success of Love Hina and thought to replicate it. In any case, it ruined the anime and made a really interesting fantasy action romance into a generic lightweight. At least the girls were cute, as intended, but that's just not good enough.
Junai Sensation and Sakura Zensen are shoujo comics, so of course they never got an anime. These cutesy young girl romances are great for mainstream readership but not for hardcore anime fans, so they're always tossed aside. What's annoying is both series are serious takes on love, death, and the meaning of life, but just because they have bubbles in the background no one takes them seriously. These stories are so much better than 12-Sai, but no one knows about them.
Kodomo no Jikan had the same issue where the manga was too illicit for the animators to portray correctly. They constantly tried to tone down the sexual nature of the show and ultimately cancelled it midway through the manga. Just like how Oreimo wasn't allowed to show a successful happy incestuous romance at the end, Kodomo no Jikan refused to show the successful pairing of an older man with a younger woman. When you censor the central premise and purpose of the story, you lose a lot of its quality. Go figure. Even with that handicap the Kodomo anime is impressively good.
MIX will likely get an anime someday. As a sequel to Touch, it would be a surefire success. It's probably just too short to animate as of yet. But if they do neglect this manga by the godlike Adachi, it would be a terrible loss. The art is beautiful, the romance is heartwarming, and the characters are so likable and down to earth.
Omoi, Omaware, Furi, Furare has the same problem as Strobe Edge with the added complication of it not being done yet. If Strobe Edge couldn't get an anime, Omoi has no chance, more's the shame.
Ad Astra per Aspera is too short to be worrying about an anime right now, but I wouldn't be surprised if it got one in the future.
The Angel Beats manga should have been given an anime by now. Angel Beats was a hugely popular anime, and no doubt a prequel would sell well too. Key works are so commonly ignored, though, that this is just par for the course. Where's the Kud Wafter anime? Or the Tomoyo After Story anime? Instead we get original junk like Charlotte which no one wanted or asked for.
Vinland Saga is probably just too violent for anime to try to cover. Unlike all the sexual censorship issues of the previous works, this has a gore and death issue animators like to avoid. The realism behind the violence makes it that much worse, since this is a historical fiction. It isn't like Re:Zero where you can just laugh off the guy's gruesome deaths because he revives shortly thereafter. There's also the problem where the manga is massively large but never goes anywhere or does anything. The protagonist is a freaking pacifist of all things. It's kind of like watching grass grow, not exactly thrilling entertainment for an anime studio to take on.
Slow Step only had a few oav's, so the story was rushed and full of holes. It even brought in filler just to keep the rotten hulk together. If they did this anime again, this time with faithful devotion to the source, it could be a wonderful anime. But what's the use when they still haven't adapted the second half of H2, the end of Touch, any of MIX, etc. . .
The 19th discrepancy between my great anime rankings and my good manga rankings is just a clerical issue. Nanoha Vivid is one of my favorite mangas of all time, but Nanoha is mainly an original anime. As a result, Nanoha doesn't count as a 'manga-based anime,' despite being in my manga rankings. I sure wish they would make a second season to Nanoha Vivid though. They ended it in such an awful spot.