I'm not quite sure who said this to me, but to paraphrase, a lot of times anime only serve as advertisement for the original work it was derived from. I would mostly agree to it because if you ever read the original form, whether it is a manga or light novel, first and then watch the anime adaptation; you would be justified in saying the anime is a worse platform for delivering the story.
From the pacing of the anime to it simply not feeling complete, the conversion from manga to anime has its problems that are quiet easily spotted out to anyone that has seen a decent amount of story driven content. I just want to point out a few anime that have fallen into the categories I listed above and to layout how it effects the quality of it's adaption, if at all.
Rushed to Completion
Anime that end up being that average of 11-13 episodes and feel like they were rushed would fall into being too short.
Kodomo no Jikan
Kodomo no Jikan is probably one of the more frustrating cases for me. I tend to tell people that this is one of my top anime, which is wrong since the manga is what I mean. Its anime conversion is extremely dumbed down and blatantly only really focusing on the ecchi parts. The manga does have its more sexual part sprinkled in, but it also has a deeper story with all three of the main characters, Rin, Mimi, and Kuro, which are never touched by the anime.
The weird thing about this series is I was first introduced to it via the anime and later read the manga. I don't usually jump to manga after watching the anime, but for some reason I ended up reading and catching up to the more recent issues quite quickly. So really in this case anime as a form of advertisement worked for me picking up Kodomo no Jikan manga, but the anime wasn't a great example of the plot from the manga.
Usagi Drop
Very much the stronger anime in this category, Usagi Drop focus is mainly on the relationship between Daikichi and Rin. Without actually having read the manga I do know that the anime did a great job presented the core of what the plot was, but it was far too short to end on eleven episodes.
I do also want to read this manga but I haven't really found the time to do that right now. However, I have heard that relationship between Daikichi and Rin do complicate. I'm not sure what exactly it is, but that does seem intriguing and the anime was good enough for me to want to continue my investment with the series.
And that is another successful advert for the manga, at least for me, I guess I really like josei types despite being a male.
Incomplete Story
With no real resolution, and the anime feels like it is missing a bunch of information would be put into incomplete.
Skip Beat!
When I finished the Skip Beat anime it felt like there would be another season. This was back in 2009 and now three years later we still have a non-concluded anime.
I've held off reading the manga because I thought there was going to be a second season, but now remember the series after such a long time I don't think there will be one and that is annoying. It was quiet a while ago since I've seen Skip Beat, but I was really into it back when I first watched it. I think I would still be interested in this type of anime, so if picking up the manga would be the only way to continue its story, that would be something to consider.
So really as far as I know this is one that isn't clear if it worked on me since I've postponed reading the manga in case of a new season of Skip Beat. However, I do want to continue the plot of Skip Beat and at this point I pretty much forgot the story for the series.
Deadman Wonderland
Deadman Wonderland is one that falls into incomplete and short since it does feel like there is more to the series and it ended without really feeling like a solid stopping point. Deadman Wonderland was one of the first time I was introduced to the survival game scenario and these gory types of anime are also something that peeks my interest.
I've heard that the manga does a better job in laying out the story more than the anime since most of it felt rushed into those twelve episodes. With that you can see why average length anime, with a concept that can be expanded on a good amount, don't mix too well.
I did however, put it into incomplete or unresolved story because the way it ended felt like they were teasing a new season that hasn't come. It's really strange when anime do this because I usually end up wait for the next season rather than reading the manga. And as I've explained is counterintuitive to the companies that might want to use the anime adaptation to advertise the original work.
Weak Explanatory Power
Stories with a lot to cover and anime that usually doesn't get the details down that well, would fall into weak explanatory power. Most anime could fit into this category, but the ones with a ton of backstory become so glaringly obvious, it overshadows the many behind them.
Shinsekai Yori
It was only a few weeks ago I made a post about Shinsekai Yori being some what hard to follow. I can't really say for sure that this anime isn't as well explained as the manga or light novel, but at times it does get a bit overwhelming with the detail of its lore.
Without getting into too much detail, since I already have a seperate post for it, it is pretty safe to say that the light novel is probably the best place to get all the detail you need for the series. Having only one way to express the finer points, makes the focus more on the writers indent rather than mixing the media and seeing which would be a suited for certain sections.
Wrapping Up
Using anime as a platform for driving more people to read the original work isn't really something I could say is blatantly good or bad. Rather it is something that is happening and in some cases, we get a good amount great anime out of it. We may also get bad ones, but the flow of an idea to manga/light novel to anime is something that need to continue even if the anime's only purpose was to serve as a platform for getting the word out for an interesting manga.