One of the main problems I had was I didn't feel like the characters motivations were sufficiently justified or explained. I never had a strong grasp on why everyone had such a strong reaction to Arty. You either worshipped him or wanted to kill him (or both at the same time). It sort of rang false that he would elicit such strong feeling from the other characters because nothing in his portrayal elicited such a strong feeling from me.
Hmm. This is actually an interesting question. I think part of it might be an indication of Olympia as an unreliable narrator; given that the entire book is told through the lens of someone who did worship Arty, as strongly as anyone in the book did, things might be colored towards fanatic opinions. I can't give you a chapter-and-verse argument for it, though.
Another reason, I think, is keyed into the general attitudes of most of the characters in the book. Most of the Binewskis, and most of the people around them, are remarkably and bizarrely sanguine about all of the weirdness that surrounds them; a crucial part of the plot, I think, is that most of the characters are so trapped by the weirdness or have no other paradigm and thus end up just drifting through with it. The Arturans are the phenomenon writ large -- people with so little in their lives that they become hypnotized by a carnival freak. About the only characters in the novel who
are introspective and forceful about their lives are Arty, Olympia (later on), and Electra (also later, and to a lesser degree). Arty is the first one to really think and speak about the condition, and so he's a catalyst for obsession.
(The only character who really competes with Arty in this regard is Miss Lick, who in many ways is his opposite... but saying any more would be spoiling, I think.)