Liliana, Heretical Healer is a Legendary Creature costing 1BB (1 colorless, 2 Black) to cast. She has a 2/3 body with Lifelink. To transform, she requires that a nontoken creature you control dies. You then exile her from the board and return her as her flip side, Liliana, Defiant Necromancer. In addition, you also get a 2/2 black Zombie creature token on the field. This is a nice bonus, considering that you get a blocker to protect her as a planeswalker.
According to the lore of Magic the Gathering, for someone to become a planeswalker, their "spark" must be ignited by an ordeal. From Liliana's backstory, that Zombie is supposed to be her brother who she attempted to save from death. The shock from turning him undead rather than saving him is what caused her spark to ignite. It's sad, but flavorful, and actually useful.
Liliana, Defiant Necromancer is a planeswalker with 3 abilities, starting with 3 loyalty counters. This seems to be a pretty low number due to the fact that most normal burn spells can immediately deal with her in Constructed. Because of this, you're pretty much always going to want to use her first +2 ability: each player discards a card. This is a pretty standard Liliana effect, and the same as the +1 ability on Liliana of the Veil.
Her -X ability is probably the one you would want to use the most: return target nonlegendary creature card with converted mana cost X from your graveyard to the battlefield. The inclusion of nonlegendary must be to protect against bringing back future Legendary creatures like herself that can transform into Planeswalkers. In any case, it's a useful ability.
Liliana's ultimate for the cost of 8 loyalty is a pretty awesome emblem: whenever a creature dies, return it to the battlefield under your control at the beginning of the next end step. That's a very scary thought. The question of course remains how often she will actually reach 8 loyalty and you'll be able to untap with her to use her ability.
The obvious downsides to the set-up of Legendary Creatures that become planeswalkers are that the creature side can die to standard creature removal very easily and that there's a necessary transform clause to become the Planeswalker. In Liliana's case, however, it's very easy to fulfill that clause, especially if you have a sacrifice ability already in play. That may not be as easy to do in most Constructed formats, but it is very easy to do in Commander. This incarnation of Liliana could serve as quite a useful mono-Black Commander and is being heralded as a great one for the Tiny Leaders variant of Commander (50 card decks with no cards with a converted mana cost of greater than 3.)
These Origins planeswalkers are definitely one of the more awesome things that Wizards has done in Magic, and how they made their backstories work mechanically is pretty awesome. Taking iconic planeswalkers and allowing you to use them as Commanders is even more awesome, and while that wasn't the primary motivation behind printing them, it's going to make them highly sought after. It will be interesting to see how cards like this play in Standard and Modern, as they will be legal in those formats. Very exciting!