It feels different near the Shire....

[hobbitlass]

I like the points brought up so far. I think the difference lies in what is happening in the world around the characters and how much they know.
In "The Hobbit" and "Smith of Wootten Major", the world as a whole is fine and everything is running smoothly except what the major characters are experiencing. Therefore the elves and other people they encounter(to me) are more light hearted and gay.
For the beginning of FOTR, the hobbits didn't know the full extent of this "ring business" and the adventures they come across till Bree have nothing (major) to do with the "ring business" and therefore still maintain that light heartedness. Afterwards, everyone for good or ill is spent on the outcome of the end of the world (as they know it) so they are pre-occupied and lose that light-heartedness.
I hope I said my point without confusing the heck out of anyone.