Unremarkable male high school student accidentally enrolls at high school for monsters. Mutual love at first sight with moe vampire girl. Shounen harem develops with other cute monster girls (succubus, ice maiden, etc.) throwing themselves at protagonist for trumped-up reasons. Shounen, his harem, and a voyeur werewolf sidekick join the school newspaper club.
Production values are surprisingly bad. Character designs are stale, detail is often lacking, backgrounds are slap-dash, often the drawing seems crude. Cheesy tricks are used to reduce the cell count.
Most episodes are self-contained. Each episode has one fight, at the end. Near the end of this 13 episode series there are a couple of longer story arcs. They are not any better than the single-episode stories.
One remarkable aspect of Rosario + Vampire is the sheer quantity of ecchi. It must have more panty shots per broadcast minute than any show except the Aika series. Immense, mobile breasts are also very much in evidence. And some shoujo-ai, briefly. Which is just as well, because it involves a loli.
There is one other aspect of this otherwise uninteresting show that is unusual enough to mention. In shounen romance the boy usually has to fight for the girl's affections through the entire production, to finally win her at the end. Example: Maison Ikkoku. In shoujo romance, on the other hand, the match is often made in the first episode, and the drama is generated as the lovers resist threats to their relationship, such as rival suitors or disruptive events. Example: Fushigi Yuugi. Rosario, despite being a shounen show, has the latter structure. I don't think I've seen this before.
Executive summary: Rosario + Vampire has little to recommend it.