The biggest complaint I've heard is that people are upset that director Peter Jackson broke The Hobbit into three movies, and that he apparently did this to "make more money" or "milk us for all we're worth" or to "extend the hype machine." And while those things are happening--whether by design or default--breaking An Unexpected Journey into three movies is not doing The Hobbit as disservice. At least as far as I'm concerned.
I've heard and read so many reviews that state this break up of the book made the pacing of An Unexpected Journey too slow. So I went into it expecting long drawn out bits that would make me roll my eyes and squirm in my seat. Instead I had almost no problems with the pacing, and when I did think the movie was going too slow, it was because of other factors. For instance, when the adventuring party finds itself in the goblin caves, I couldn't wait for that to be over, but it was mostly because I didn't like the overly comical interpretation of the goblin king, in personality and CGI-ness.
In the end the pacing felt just like the pacing of the Lord of the Rings movies. There was certainly plenty of action, and there were certainly slower bits (but not too slow or too long) that involved the characters actually talking (oh no, not talking!), which, ya know, moves the plot forward and helps you feel connected to the characters.
The other biggest complaint, which is the biggest complaint whenever a book is made into a movie, is the plot and/or nuance changes in the adaptation. But I have two things going for me that reduce my anger towards plot changes: 1) I don't often remember plots of books I read in minute detail, but rather in general arcs and feelings, and since I don't remember minutiae I'm not upset if it changes; and 2) I decided long ago to do my best to look at adaptations as their own entity. They are different mediums and must be treated and consumed differently. To be clear I do get attached to plot points and details that I hope to see in an adaptation, and I am disappointed if my "pets" don't make it into the film, but I try not to let minutiae ruin my experience. However neither of these were really a factor with this adaptation, because I didn't love the book in the first place (I know, I'm a monster), so I ended up liking the movie much more than the book.
Now that all of that is off my chest, let's move on to the part where I talk about whatever I want with no through line and little focus.
I can't imagine anyone being more perfect as Bilbo than Martin Freeman. Since Mr. Freeman has a solid background in comedy and drama, he was able to play Bilbo's fussiness and inexperience as an adventurer without making Bilbo overly naive or ridiculous. His facial expressions, body language, and perfectly timed stutterings endeared Bilbo to me in a way that I didn't experience in the book.
The other best bit to me was the introduction in the movie. It not only gives a lot of history that's needs to understand the movie's climate and the character's motives (without being boring), but it connected us back to the LOTR series. In fact, I had a couple of aha moments that brought some things full circle for me.
1) It was weird to see Gandalf as Gandalf the Grey, I just got used to Gandalf the White at the end of the LOTR series.
2) It was very hard to not immediately see Saruman as evil because of LOTR. And it didn't help that there were devices that could have been implications or foreshadowings of his turn to the dark side.
3) I loved the solemn song the dwarves sang and that it became the theme music throughout the movie.
4) I loved that many of the actors playing dwarves weren't directed to change their native accents, so we had dwarves from, at the very least, Scotland, Ireland, England, and New Zealand.
5) I was also very pleased with Richard Armitage's performance, even though I already knew he is stellar at playing intensely serious and overly proud characters. (See North and South .)
6) And just for fun, this video of Martin Freeman and Richard Armitage.