Preface: I'm a huge, huge Tolkien nerd. I enjoy the LOTR movies, they are quite good - as movies specifically - and pretty decent - as LOTR movies.

The Hobbit was completely horrible, and I will not be watching the two following movies.

So, if you haven't seen it yet, you shouldn't read on if you don't want SPOILERS:

Big thing about dates that I mention: the movie is wrong (I think I remember it saying that the sack of Erebor - the Lonely Mountain - is 60 prior to the start, but I could be just remembering the 60 years prior from the flashback thing at the very beginning of the movie). Erebor is attacked and sacked by Smaug in 2770. The Goblin War is 20 years after that. And then there is another 150 years before the start of the story of The Hobbit.

As darthspudius mentioned earlier there are a bunch of things added into the movie that are not directly in The Hobbit as a book. Now, while some (key word: some!) of it is relatively grounded in actual lore, a large amount of it is actually completely made up.
The best example of this is the Azog ("The White Orc") subplot for Thorin. The battle that is shown in the movie which occurred at the gates of Moria: that happened, true, about 150 years before the Hobbit takes place. That is correct. And the orc chieftain there was named Azog. And he did behead Thror (though it isn't in battle: Thror goes to Moria with one companion, Thror enters Moria by himself to scout it out and is captured, killed, and beheaded by Azog - this is what sparks the Goblin Wars). Thorin does fight in the battle (sort of) as depicted, and he does lose his shield and use an oak branch as a shield - hence Oakenshield as his honorific. He does not actually fight Azog though: Dain II, who is Thorin's cousin I think, (who shows up at the end of The Hobbit in the Battle of Five Armies leading the dwarves from the Iron Hills, he then becomes King Under the Mountain after Thorin is killed) fights and kills Azog.

So there is a pretty big difference. I understand why they did that though - so they didn't have to try and explain that back story and then introduce another orc chieftain (Bolg - Azog's son) who shows up at The Battle of Five Armies as the huge antagonist. Having Azog chase the company around though is ridiculous and stupid. And just an excuse to have more pointless fight scenes in the movie (and to help stretch the time out).

Another example: Dol Guldur, the ruined castle in Mirkwood. So, Gandalf already knows by the time that The Hobbit takes place, that the "Necromancer" there is Sauron. Gandalf knows this because years previously (between 2841 and 2850, which is about 90 years before the Hobbit takes place) he ventured into the dungeons of Dol Guldur and found Thrain, the father of Thorin, who had sort of gone crazy after the death of his father and the end of the Goblin Wars. Thrain was crazy enough, and had been tortured enough that he didn't actually know who he was, but he trusted Gandalf enough to give him the key and the map to the side door of Erebor (which Gandalf has in the movie and gives to Thorin).

In escaping from Dol Guldur he basically figures out that Sauron's spirit (not yet re-embodied, though the Witch King (Lord of the Nazgul) may be fully bodied at this point - it is unclear) is living there and that is why bad things are going on. He knows about all this long before Radagast tells anyone (and Radagast's portrayal is also a bit ridiculous in the movie), and while the White Council (Saruman, Galadriel, Elrond, Gandalf, possibly Radagast) knows what is going on before hand they don't make any move because, on the counseling of Saruman, they are being cautious, and don't think that Sauron can regain his strength.
The movie shortens this whole timeline - has Gandalf look super uninformed about things - and also makes him look really weak in the Council. All things that are problematic, at best. Also, the placement of the Council meeting in Rivendell (let's put aside the teleporting Galadriel thing for the moment as well!) is probably wrong - and it certainly didn't occur at this time. Gandalf attends a council with them when he leaves the dwarves and Bilbo after resting at Beorn's house. This grouping of them go to Dol Guldur and drive Sauron and the Witch King out and they flee to Mordor (Sauron basically tricks them though and they think he is weaker than he really was at this point - meaning he probably had regained his body, and most of his power: without the Ring obviously).

Besides lore things that I can continue to critique all day though the movie itself has horrible schizophrenia: it doesn't know if it wants to be dark and deep and brooding with lots of fighting and sadness and death - or if it wants to be comical and light hearted and silly even when bad things are happening.
Examples: The whole Azog pursuing the dwarves from the moment they leave Bag End is super dark and dramatic -and allows for chase scenes and fighting galore. But at the same time the battles themselves are super unrealistic and silly looking. A friend who I saw the movie with last night commented that the fights "look like a video game" (she is not a gamer). And I agree, in all the negative ways that game fights can look.
The Goblin King is supposed to be scary - but at the same time is a farcical character. Particularly in the way he dies (saying "That'll do it." when Gandalf slashes his (huge) belly), and then in another moment of physical comedy his body falls on top of all the dwarves - for laughs.
But then the overly dramatic fight between Azog and Thorin on the cliff's edge, and then Bilbo jumping in, and then Thorin being seemingly "dead" is so dramatic and over the top in the opposite direction that the movie is pulled in two.

As a final criticism I would like to point out that Peter Jackson still has no understanding of what a metaphor is:
I do concede that the literal way that The Hobbit is written there are "stone giants" who are throwing rocks at each other for sport in the mountain pass. However, I see this whole little diversion thing (even including the dwarves making a comment about not wanting to be made a "football" of the giants) is a story telling technique, and ultimately metaphor for a great big old thunder storm (a "thunder battle").
This is similar to how Jackson doesn't understand what subtlety is when reading LOTR, regarding the "wings" of Balrogs. Balrogs. do. not. have. wings. The specific line in FOTR is referring to a great shadow (which the Balrog is made of an, and can create) which is "like wings" and looks like wings.