From UbiSoft's official Assassin's Creed Unity site:

To show our appreciation for your continued support, we’re making the upcoming Assassin’s Creed Unity Dead Kings DLC free for everyone. For Season Pass holders, we will also offer the choice of one additional game from a selection of Ubisoft titles for free.

The list of games season pass owners can get is pretty good. Far Cry 4, The Crew, Watch Dogs, and Rayman Legends are all serious games that are better giveaways than companies tend to do in these situations. I don't know how well this will ease Ubi's relationship with games, but at least those who felt wronged are getting something back.

This is also much better than what DriveClub offered gamers who suffered through their botched launch.