For Howl-O-Scream, they are adding new shows, a new maze, roaming hordes of scare actors throughout the park, along with other surprises. The nice thing is that they are adding the new maze to areas normally off limits, so the new maze will be in addition (will now have 6 mazes). Also of note, Fiends will be back this year, but the staff specifically said that it is not for children.
For Christmas Town, they're adding a new show, along with the opening of Festa Italia for the first time during this event. This will add an additional 1 million lights for the event, so it will now be over 6 million lights for Christmas Town. :)
And finally, for 2013, they're going to be doing a Food and Wine Festival for the first time in the spring.