That should have stayed a manga only!

Although I'm an avid manga reader and not enough of an anime watcher, I can't really place any opinion on mangas that should've stayed as mangas. Only a few shounen titles come to mind, like Naruto, Bleach, Rurouni Kenshin, but that's because of complaints about filler episodes not contributing anything to the anime as a whole. Especially after the Shishio arc in Kenshin anime, where nothing seemed to happen. The Revenge arc is really where the steam picks up in the manga.

Another thing that is common in manga to anime transitions is cutting the anime off at a point where it still continues in manga form. Many animes do this, and sometimes loyal manga readers will think it pointless to watch the anime when you can get the complete story and picture by reading the manga. On the other hand, watching the anime might be a better platform for many people to become fans of the series and thus begin reading the manga.

While writing this I finally thought of one: Angel Sanctuary. They made it into a two part OVA which was minimal time to explain anything. The entire manga is 20 volumes long, if I remember when I read it, and there were too many characters and plot twists to even squeeze anything relevant into two hours. Art is pretty, yes. OVA is a mind boggling NO.

Jam it back in, in the dark.