The Light Novel Quality Boom
I’ve noticed that recently a significant majority of the anime series I’ve enjoyed most are based on light novels—Spice and Wolf, Baccano!, and Toradora, to name a few (the first two of those were both winners of the Dengeki Novel Prize, in fact). Novel-based anime is nothing new—Vampire Hunter D and The Dirty Pair are both based on novels. But, when I look at older stuff, a much larger percentage seems to be based on manga than is the case now. And even if the ratio hasn’t changed, I can say for sure that the proportion of good prose-based anime to bad is markedly better than that of good manga-based anime.
Which, if you think about it, makes a lot of sense; assuming most up-and-coming manga artists write their own material (which seems to be generally if not universally true of Japanese mangaka), someone who is a good artist but mediocre storyteller is a whole lot more likely to get published than someone who’s a good storyteller but mediocre artist. In contrast, if you’re a novelist, the only thing you’ve got to be good at is spinning a yarn.
So, since most anime draws from either manga or novels as source material, the anime that draws from manga is, on average, going to be drawing from a pool with a significant percentage of things that got popular due to what they look like more than their story. Novel-based anime, however, is almost by definition going to be based on something with a worthy story. And, since you’re going to have a team of professional artists working on the anime either way, the novel-based material is going to have the upper hand in everything but a foundation in visual storytelling. Light novels are an additional advantage versus “heavy” novels, in that they tend to be shorter and less intellectual, which is going to translate more readily into a TV format. They also have accompanying illustrations already extant, so there is some visual identity to build off of.
Obviously there are plenty of great manga-based anime series, and novel-based duds (say, Kanokon), but on average it makes sense to me that novel-based anime has the statistical upper hand when it comes to telling a compelling story. Which is why I’m glad to see more and more anime based on light novels—bodes well for the future of the medium, and industry.
In terms of hard numbers, skimming through my personal list of five-star anime, I see four based on manga, six original concepts, and  a whopping eight based on novels of one sort or another. That compares to zero based on novels in the 2-star range. My personal top ten contains two novel adaptations, three manga adaptations, and the remainder are anime originals—less impressive, but still skewed heavily away from manga.
As an aside, what the heck is a “light novel,” anyway? The page count and physical size isn’t significantly different from “heavy” novels, and it’s not that unusual for Japanese novels to have a few illustrations, so I suppose it has more to do with the general style of a series of shorter stories spread across a number of books, as opposed to single, tightly-packed, standalone stories. You could also make the argument that light novels are targeted at a demographic and style usually served by anime and manga, but that’s a little unfair to the medium (not to mention rather meaningless, if you factor in josei and seinen adult-targeted manga—all you’ve ruled out as an audience are old-timers). The genre is probably most closely paralleled by the Borders category “young adult” fiction, though it’s not a perfect match.
If you believe the US publisher of the Boogiepop novels, Boogiepop is what got the light novel trend started back in 1998, and even if not there’s another great series based on novels rather than manga. That date also aligns with the increase in such novels and their anime spinoffs since.