This has been the number one complaint of the first film, and given the time frame the sequel will encompass is significantly more packed than in AUJ, the film will likely move at a faster rate with less filler and more truthfulness to the book.
2.) Deus Ex Machina
A problem that is pretty much in every ME movie, but is more prominent in AUJ. In the chunk of the book that DOS covers, Gandalf departs from the dwarves' journey, making it so the main characters (including Bilbo) will have to fend for themselves. Gandalf won't be able to come in every five seconds to save the company, resulting in more involving action scenes. Not to mention, no eagles.
3.) More seriousness
Another problem with AUJ was out of place humor, resulting in tonal imbalance. However, the next few chapters of the book become less lighthearted, and more based around the company's misery. Hopefully this will reflect on screen.
Pretty self explanatory. If done right, Smaug could be one of the most impressive CGI dragons ever put on screen. The Gollum of The Hobbit! They teased us plenty with him in the first film, hopefully it'll pay off.
5.) Bilbo's character arc
In this point in the book, Bilbo starts to grow significantly--from freeing the dwarves from the spiders, to leading the badass barrel escape, to facing off with Smaug, Bilbo's part in the story will be of greater significance.