I did not like the addition of Adam and his sister. In the book, the vampires were more of a faceless threat and I liked that. They were puppeteers using the historical characters to do their bidding. The plantation plot was much better played in the book, as Abe was lured to the plantation in hopes of killing Jefferson Davis.
I haven't read the book yet, but I did feel that having the vampires be the baddies rather than regular humans who were practicing slavery (that's not bad enough?) was a bit off-putting, as though human evil has always been the fault of vampires throughout history. That lets humans off the hook way too much! I wasn't sure how to make the connection better, but refraining from personifying vampires sounds like a good idea. Just keep them a nebulous evil force out there, and make the conflict be essentially between humans.
The ending from the book about Henry and Lincoln, wow. I can see why they decided not to do that. That's just too nutty to work in a movie.