So about this "Hobbit"...
I'm not complaining about taking liberties with the book -- every movie-from-a-book does that. There are some slight spoilers in the following, but most of them are either well known or pretty predictable so I make no apology.
Yes, they added a lot more fight scenes, because an action movie needs a lot of fight scenes. Yes, they renamed the "goblins" of the book as Orcs, because they already had Orc costumes in the Weta warehouses and it's simpler than explaining the difference. Yes, they inserted Legolas into the movie, because Orlando Bloom. And they inserted hot-elf-maiden, because otherwise there would have been no female characters at all, and because "Hunger Games", and "Brave", and "Hunger Games". And they had Stephen Frye, who might have been good as a dwarf but those parts were all taken so they had to write a sort of schizophrenic Stephen Frye character into the story.
What bother me are the inventions and insertions that make no sense at all, except as an excuse for a cool visual effect or star fan-service. Many of the fight scenes are preposterous in the same ways as those in "Pirates of the Caribbean": no, there's no reason for the characters to do this, but wouldn't it be cool to see them do it?
A small example: Legolas is fighting some Orcs with a knife in a small room, then pulls out a bow and shoots one at a range of a foot. Why? Five minutes later, he faces the baddest Orc of them all at 10-15 yards range, so this awesome archer pulls out... a sword. WTF?
A larger example: you're in a dragon's lair, with your life threatened from second to second. So naturally you find "the forges," which haven't been used in fifty years (and you seem surprised that "they're stone cold"), light them with a handy dragon (and no visible source of fuel), and start an industrial manufacturing line in order to melt a couple thousand tons of gold so you can go surfing on it, then cast a 100-foot-tall gold statue (having presumably built the mold in a few seconds off-camera) which, as soon as everybody's gotten a good look at it, starts spontaneously re-melting so you can try to drown a dragon (who lives, eats, and breathes gold and fire) in molten gold. Gee, who wouldn't think that was a good plan?