movie done so surprisingly well (I suppose it's that I dislike the title, which, notice, I'd like to leave unsaid -ehsisterhood -ehtravelling -ehpants) I thought book would not be as good. but it is quite good. and after reading it, there are two differences I would like to have seen in the movie:

-----a dark good-looking Eric. he did not strike me in the movie at all, so I was happy to find his very different description in the book as part Mexican, with longish dark hair. With a different actor as Eric, Bee's interactions with him could have interested me much more.

-----a stronger actress as Lena, more depth, more solidity so that you can see her as the mother figure going to Bee at the end of the book. Alexis Bledel (Rory) seems weak and timid rather than inward and careful. and not beautiful. she's the weak element in this as in Gilmore Girls, both otherwise so excellent that I love the idea of how good each would be with a great actresses in her role.

all the girls are pleasing to watch _except Alexis_! Blake Lively as Bee was quite striking, charismatic on screen; Amber Tamblyn might be the one I find actually the prettiest, as well as most likeable; America Ferrara was good, believable.

I guess I'd like Tibby the best of the characters and oh Bailey. but the stories of Bridget and Lena especially in the second book are the ones I most...these are where heartbreak...

Bridget gorgeous and bold with the faultlines/breaks of loss, Lena inward and lovely (inthebook -darnitRory), Tibby smart and witty, and Carmen emotional and vibrant (least apealling to me in book and movie, but because of my leanings, not because of any weakness or less likeability in the character)

a group gives you relative identities. "she's the rebel / she's the sporty star / she's the artist / she's the livewire." ah relation. community.
comparison, contrast <- variation, versions.

so of course I have no general idea that movies should avoid changing source material.

but I am with this commenter on IMDB:
-I was wondering why they changed things that didn't need to be changed. Like Kostos lived in New York and not in Chicago. And Lydia's mom died not her dad and the place there were going to have the wedding double booked their water main didn't break. I don't know why they changed this kinda stuff.
right, why bother? I wonder.
and a more pointed alteration - the moviemakers must have had an actual motivation, a line of reasoning for this, right? Why change the Lena/Kostos story into one of forbidden love? made it more a cliche, made this part of the movie less real-seeming and more comic. seems to me it would have played better as it was written in the book: the encounter at the Olive Grove, the misundertanding, the next encounter. they could still have written in additional screentime of scenes with Lean and Kostos together... ?
let's see if I can find any information..

also on IMDB:

-Special recognition should be given to Jenna Boyd as 12-year old Bailey, who believably transforms one teen from misogynist to humanist. This little actress has the chops to win the Oscar someday.
- I live in Bethesda(well Kensington, but I go to school in Bethesda so it's like 8min away) where the book was placed . sweet Kensington...