Clearly this question only arises because I did not like it very much. Bad (but not offensively bad like, say, Air or The Jennifer Morgue or The Execution Channel).
As someone who is keener on SF and detective fiction than lit-fic, I wonder if I am allowed to judge it by the criteria of either? For me, the alternate world does not really work. There are a few hints that there are other differences apart from the Jews being allowed to settle in Alaska and the collapse of Israel in 1948 e.g. Manchurian astronauts are mentioned, but it is hard to believe that if there were a Jewish district in Alaska about to revert to US control with the occupants having no right to remain in the US, that it wouldn't at very least be extended, and the book does not in any way justify this.
It is almost a cliché that detective stories feature a detective with some wacky attributes in some wacky place (e.g. a Morris dancer with Tourette's syndome working as a chiropractor among the Maasai who finds himself solving back-pain related crimes in Tanzania), and you expect a lot of 'sense of place' from detective novels. You would, I guess, expect MORE rather than LESS of this description about an imagined place, but actually you get less sense of place of Sitka than you do, say, in Hillerman or Rankin. Paradoxically, in the interview mentioned below, Chabon says he realises he needs to do this, but to my mind he fails. There are a couple of pages where Sitka comes alive, but on the whole the place and the prose are very flat. In the interview he also said he did a lot of planning of the history. I think he would probably argue that he is deliberately not telling the reader things that would not be in the novel if Sitka were a real place.
Chabon is obviously over-pleased with himself, particularly having come up with a Yiddish word for 'mobile phone' he uses it rather a lot. He says the original genesis of the book was a Yiddish phrase-book, but of course we don't really get any sense of this because the book is written in English. It was on the nth mention of the Shoyfer that I turned against the book, I can tolerate many things until I am sent over the edge, at which point I being to relish finding fault. A much better (non-fiction) book on Yiddish is Lansky, Aaron, Outwitting History: How a Young Man Rescued a Million Books and Saved a Vanishing Civilisation, Algonquin Books, Chapel Hill, 2004, ISBN 1-56512-429-4.
Here is an interesting link on NPR about the story of the possible Jewish settlement in Alaska http://www.npr.org/templates/story/story.php?storyId=10150792 Various of the people mentioned in the NPR piece are mentioned in TYPU, mainly as having roads named after them. As is often the case, the reality sounds more interesting than the fiction. And here is a long interview with Chabon on Fresh Air http://www.npr.org/templates/story/story.php?storyId=9974891