Note: the following comes from a video review, links on my profile page.
One of the first noticeable things here is the quick change of genre; mostly taking a couple of episodes, they delve into entirely different worlds, kind of like Chrono Trigger, which then ended up resembling different styles.
It starts off in a Baccano-esque train, which makes one think it's possibly a neo-noir, although even within this setting there are odd traits, like a traditional Japanese dress within a Siberian surrounding, and what seems like a stereotypical magic user (two sort of constants with variations throughout).
While there is always an entirely different type of environment, there are still similarities, besides the protagonists. What is interesting (that some people don't seem to appreciate or understand) is how the whole of this anime is like a combination of Inuyasha and Lain, with added innuendo. In thirteen episodes it certainly managed to include a variety of narratives and visual differences. It could have combined elements even more explicitly, but the subtle approach to it is interesting too.
The point of this anime isn't the ecchi at all, no... how much hentai out there has the same average rating, though? For some people it seems having everything else also interesting makes no difference... To Love Ru is how popular, again? They have things in common; both attempt sci-fi, except in TLR it is almost deflated. Lala keeps inventing gizmos and the only thing the narrative comes up with is the same exact comedic routines invariably involving the protagonist falling, with inevitable coincidences. Lala in Yami happens to be an AI in a spaceship that cares for children for one particular reason... and yet the latter is what the average viewer rates less, really? Yami has so much depth to it, TLR tried to have bits of romance too, only to fall flat on its face (literally, often), whereas the romance in Yami can be funny and deep at the same time, with added existential complications.
The ecchi, in a sense, is only there because there is also everything else along with it. Ecchi shouldn't be the point of an anime, just like the sports genre shouldn't ideally focus exclusively on that, and instead be merely an accompaniment along with characters' lives. If there is only ecchi, of course, it would practically be hentai, and that is what TLR very nearly is sometimes. Whereas with Yami, even in such scenes, the narrative is making itself cohesive, and not a moment is wasted on capricious nudity for its own sake.
This is when a story is powerful - when, in a scene that could evoke merely lust, there is also a sense of inter-dimensional love, of a chance that something could be lost. Anime that would not settle with only a single relationship, but includes countless analogies of what that relationship could be, of what a cyclic universe could contain. This is when animation tries to do a lot, and ends up doing it, on average, well and visually interesting.
