I havent seen the movie yet but did read all 600 or so pages of the novel couple of months ago . I figure that there must a lot of lore missing during the movie production.

Prob best not to read the latter if you havent seen it (not really a spolier but shrug)

1. The addendum in the book explains the history of the elves, men, dwarves etc . very well though pretty wordy (not surprisingly P). Basically it's a return to the lands out across the sea from which the high elves originally came from. No real explanation was given why Frodo was allowed to go though his role of the Ringbearer was prob the key. Gandalf is also from a different land out west but has a ring in his possession so he goes too .

2. Hobbits are from Middle Earth and so weren't allowed to go. Also, from the rest of the book, Sam and the other hobbits have other roles in the Shire post-Frodo leaving. I guess the movie doesn't deal with that.

3. The undead hoard only existed because they failed in their original promise to honor the king and defend the land from Sarron's forces way back before the trilogy begins. Figure that doesnt really include a full assault on Sarron so they fulfilled their promise so to speak and paid their debt.

Off the top of my head, this is what I remem from the book (hell if I am going to read it again just for you P - besides getting it back from the plane I left it in P). Hope it helps