sorry about posting so late on sunday night, AGAIN but i didn't have access to a computer
what i noticed in part II was how Sal's opinion of Dean has changed so much. He used to look up to him and want to be wild, carefree and live in the moment like Dean, but now he's changed and wants to settle down with a family. I think that shows that the trip to the West DID change him even if he feels like nothing happened. He found what he wants to do in life which is to settle down with a family.
But then when he, Dean and Carlo take off again, he realizes he wants to be on the road again. he likes being on the move and going different places. It's also like he's leaving everything else and all his problems behind, taking off on the road.

We also see a different side of Dean. before, he was kind of a mystery but when they're driving back west again and he tells them his life story we see the side of him as a child living with a drunk father without a home.

I also agree with everyone that it's so much more depressing. Just the way it was written and even the characters constantly talk about sadness, especcially Sal. the whole mood is so confused and depressing, which is so different from the first time they were on the road, because even though they didn't even have a car or anything and the situation was much worse, the mood was so much lighter and they made it happy.
the characters in part two are also shown more as beats and the time period can be seen more than in part one. For example, it's the first time they really mention drugs, inthis case marijuana, which was something the beats were into.