I haven't seen this mentioned anywhere, and it originally wasn't included in the product summary for the Warlords of Draenor Collector's Edition, but in addition to the card back, it comes with codes for three free card packs. (Note: You can apparantly only redeem five free card packs per account.)
I was pretty lucky with my three packs, two epics and a legendary: Shadow Form, Avenging Wrath, and Baron Geddon!
Feel free to share your results if you also got the three CE packs.