The Mystic Archives of Dantalian a.k.a. Dantalian no Shoka a.k.a. Bibliotheca Mystica de Dantalian did not, as I feared, have a Gainax ending, but the ending was nevertheless startlingly abrupt.
In the space of the two final episodes we learn of the existence of rival supernatural libraries, meet two new pairs of antagonists and learn the real nature of the relationship between Dalian and the pink-haired girl. One might well expect that this would be the beginning of the real story, not the end of the series. If Gainax is hoping for a sequel they have certainly laid the groundwork.
Still (ignoring the half-complete Mawaru Penguindrum) I think this was my favorite series of this summer. The stories are original, the artwork is excellent and it is refreshing to see a shounen anime series with a strong, self-confident and competent hero.