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Let's first look at this from an anime standpoint, where all series (should) have equal ground. I'll only list high quality anime, since, due to manga's popularity, for every good adaptation, there's 20 bad ones...I've yet to see a bad anime adaptation of a light novel though...
Light novel anime: Haruhi Suzumiya, Spice and Wolf, Bakemonogatari, Toradora!, Baccano!, Durarara!!, Welcome to the NHK, Angel Beats!, Tatami Galaxy, Kyouran Kazoku Nikki
Manga anime: Highschool of the Dead, Mitsudomoe, Deadman Wonderland, Kodomo no Jikan, Sayonara Zetsubou Sensei, Rainbow: Nisha Rokubou no Shichinin, Fullmetal Alchemist: Brotherhood, Lucky Star, Claymore, Genshiken
In my opinion, 9/10 of the light novel anime ruminate on deeper topics more often than not, while 5.5/10 of the manga anime do.
Light novel anime: Haruhi Suzumiya, Spice and Wolf, Bakemonogatari, Toradora!, Baccano!, Durarara!!, Welcome to the NHK, Angel Beats!, Tatami Galaxy, Kyouran Kazoku Nikki
Manga anime: Highschool of the Dead, Mitsudomoe, Deadman Wonderland, Kodomo no Jikan, Sayonara Zetsubou Sensei, Rainbow: Nisha Rokubou no Shichinin, Fullmetal Alchemist: Brotherhood, Lucky Star, Claymore, Genshiken
In my opinion, 9/10 of the light novel anime ruminate on deeper topics more often than not, while 5.5/10 of the manga anime do.
Other things I've found, manga tends to stick to common ground, whether that be shoujo/shounen styles or the "sum up the entire plot in the first chapter" rule. Light novels, on the other hand, are often more innovative, sometimes to the point that it's hard to decide which genres it would fit under. They often introduce important main characters, even the love interest, many chapters after starting, something very rare in manga (the latter part, anyway).
At times, these traits can be attributed to how light novels are sometimes published in volumes, manga in chapters. Manga magazines are often specialized by genre, so its series must fit in. A single uninteresting chapter can get the mangaka in danger of being axed. This may also lead to mangaka feeling more pressured to add fast-paced action instead of slow-moving depth.
However, many light novels are published in magazines as well. So another theory to explain the differences, manga draw sensibilities from manga, while light novels draw from both manga and novels. Therefore, manga is less susceptible to change, and light novels are more open to different ideas (since novels are everywhere and much more varied).
Those are my ideas. What are yours? Debating is much loved, I'd like to get some good conclusions to this.
Last edited by Blique at 12:30 pm, Jul 17 2012