Spoilers thar be!
I don't know why Artemis Fowl 7 is so below-par and underwhelming. Perhaps Eoin Colfer was under obligation to finish to a deadline before developing any fully-formed ideas or something. Nothing in this book really satisfied me, I'm sorry to say.
Artemis is in Iceland preparing for a demonstration of a new Earth-saving eco-device when a deep space probe falls out of the sky on top of him and his friends. This results in a slow-boiling mental illness coming to a pop and Artemis' personality being succeed by "Orion". With much of their resources depleted or lost, Orion and his usual team of Butler, Juliet, Holly, Foaly and Mulch must find out the whos and whys of the devastating crash.
Colfer's first and foremost failure here is the fact that he just doesn't have enough story to sustain an epic adventure, even (and especially, I should say) one told as quickly as this. The plot is delivered in several, fast-moving, large chunks with very little time for significant development or even a single decent twist. It's all very straight-forward and as with all fast-paced novels/movies coherence is sacrificed for speed. I was frequently lost and confused as to what was going on. I'll also pin some of this blame on the fact that Colfer simply did not describe many scenes very well. On top of this the book ends with nearly all plot threads completely open-ended and multiple unanswered questions.
The globe-trotting nature of the previous books is absent. The story only superficially takes us to Iceland, Cancun and Venice. Most of it takes place inside a submersible pod with very, very little opportunity for epic or exciting scenes.
I hear that Artemis Fowl 8 will be the last one. Colfer seriously needs to get his act together and go out with a bang, not a whimper. He totally phoned this one in. The long woven threads of Minerva, Opal Koboi, Artemis's love for Holly, and his determination to heal the planet need to ALL be explored and closed in book 8.
The only reason this book exists is because you need 7 to get to 8. And for that reason ALONE fans should read it, but we've come to expect a LOT more than this from Eoin Colfer.