It's happening to lots of people, but they claim to have fixed it, at least for PCs, in the mini-patch last Tuesday. This did not fix it for me however (I'm also on a Mac). A workaround is to move/rename the cinematics directory found in :
Code:
World of Warcraft\Data\enUS\Interface\Cinematics
If you just change "Cinematics" to Cinematics1, then start the game directly without the launcher, it will work fine because the game won't be able to find the cinematics to crash you. If you're not in the US, then the "enUS" part will change to match your language and region.
Note, that you'll want to undo the naming change when you login again (on a non-raid night) or the game will then try to redownload the whole Cinematics directory and you'll have two copies. Be sure to only rename that directory - there is a second Cinematics directory which has nothing to do with the Dragon Soul cinematic problem.