Let's look at two of the most famous middle-grade series out there: Harry Potter, by J K Rowling, and Artemis Fowl, by Eoin Colfer. Both are blockbusters, both are beloved by children and adults (I hope!) alike. But in terms of content, the big difference is the series' arc.
What I mean by arc is this: from the very first Harry Potterbook, the story is culminating up to the final hoedown throwdown between Harry and Voldemort. (Hey, did anyone notice that Voldy's name means "flight of death" in French? See, this is why you should be Canadian. Sorry, random side note. Back to your regular program.) Each book in Rowling's seven-part series builds up to number seven.
By contrast, the Artemis Fowl series has no arc. Each of the characters have their own arcs, yes, but plot-wise, The Arctic Incident (#2) has almost no impact on The Atlantis Complex (#7). Colfer introduces new characters with abandon and drops them with equal abandon. Anyone ever wonder, for example, what happened to Minerva from The Lost Colony (#5)? In this way, Colfer's means of sustaining his series are through the familiar characters and a lot of action. When he stops writing the Artemis Fowl series (or when he dies,not to be morbid), it's extremely likely that the series will finish open-ended -- very dissimilar to the HP series. It was said that The Time Paradox (#6) was to be the last book, before Colfer came out with #7.
Series like Magic Tree House have no arcs, because it works that way; the series's only purpose is to educate, so readers don't mind if the characters don't age. But in general, you want your series to have an arc. My thoughts? I'm betting Eoin Colfer didn't expect for his AF books to be so wildly popular (as a professional critic, I will say that he has an abundance of clichés, adverbs and exclamation marks). Perhaps he's been forcing books #4 and on due to popular demand. And I won't lie -- count me in his fan base. I'm just worried for the direction his series is heading.
What do you think about series?