I think one of the reasons why Anime series are appealing to me is because there are a finite amount of episodes leading to a defined beginning-middle-end. However, in my opinion the majority of action oriented Animes have great beginnings, energetic middles, but end in a less than stellar conclusion.
This either stems from having too complicated "middle portions" and not being able to answer all the story threads (an extreme example is the "Lost" disease). Or the build up to the conclusion is an ever escalating series of over-the-top action sequences (any Dragonball, Bleach, type series).
In my opnion, Blood+ did a good job at delivering a conclusion that actually delivered closure to the characters. I, as a spectator, was content at what happened in the plot arc and actually was not "yearning" for more story. This is actually a good feeling in my opnion because the contrasting feeling would be a conclusion with "loose threads" that remain unresolved.
This one observation made me place Blood+ in my favorite Anime shortlist.