This DLC allows you to play as Pagans. This means you can now play as.. Vikings, Mongols, Slavs, Zorastrians, Sami, and romuvan. If you have the ruler designer there are also a couple more less fleshed out pagans to play as too. But the real kicker is the ability to play as Norse and the Tengri (Mongols).
This also adds an earlier start date to the game, however the earlier date's value can be debatable. It makes playing a viking much more fun but Europe can look really messy with all the Karlings inheriting Karling land, and it makes all the fun later game stuff like mongols take much longer to happen, which can force a game to end prematurly.
Either way, this DLC is by far the greatest addition to CK2 (at the time of this review Charlemagne has not come out yet, although that still looks like it adds less). Sure, Rajas adds Inida, which is huge, but Old Gods adds so much more, just in playable charectors and situations.