Why didn't Cecil's ghost apologize to Garth's mother in person? The book pretty much establishes that ghosts can be seen by Muggles.
Where the heck did Garth's son come from? He hasn't been born yet, let alone DIED...
Cecil, at one point, said that time works differently in the afterlife. I personally assumed that it was like the series finale of Lost, where everyone entered the afterlife at once, even though they all died at different times. That said, if I'm right, I definitely think TenNapel could have made it more clear. However, this explanation gives rise to another problem...
...If time is all crazy in Ghostopolis, how would Lockjaw know that Garth's shoes were "modern"?
Cecil points out how unnatural it is that Garth is capable of so much so soon, but the book never addresses why this is the case. Just what makes Garth so special that he's more powerful than the average Ghostopolis foreigner?
Whats with all the diffrent species? How does someone turn into one and not the other? Actually, there are a lot of problems with that. When I first read this, I hated it, but that was my biggest complaint. Is it like a lottery? Does it depend on the type of death? Explain book, explain!
Why, oh why do they have to make Benedict Arnold seem like he has a fetish for betrayal?
Flanderization done for the purpose of comedy. Whether it's actually funny or just annoying is up to the reader.