Best and worst.
Worst things they changed in my opinion: what they did with the dale character, extending the entire farm issues imto a entire season, and how they treated Andrea as well.
Best: what they did with the Governor, they manage to give him more depth and character, that was lacking in the comic book version, Turning Carol into a badass, some of the remixes that have been done with certain characters, and Daryl although I kind of consider him overrated.