Publisher: Bioware
Genre: RPG
Summary: Leliana tells the story of how she left being a bard and became involved with the Chantry.
Gameplay: There's nothing new to Dragon Age: Origin's gameplay here. There are usual opportunities to fight or sneak or find objects.
Style: There are no new locations. The loading screens and lore entries are told from Leliana's point of view, reinforcing her narration, a nice touch. Characters are introduced with dramatic splash screens. This looks cool, but I don't think it quite fits the tone of the rest of the DLC. The music is excellent.
Story: It's interesting to play this for the first time, as I have, after Dragon Age: Inquisition. It works well to enrich the complex character of Leliana and related aspects of Thedas.
As a character-driven story from Leliana's past, the main events can't change, but there are some choices in Leliana's responses and minor actions that add variety. The other characters in the DLC are generally interesting.
There's a pleasant ambiguity to the whole DLC (always an issue when you have a rogue narrating in Thedas.) When Leliana mentioned these events in the core game, for example, they did not take place in Denerim. (Clearly it does so here so the location assets can be reused, but you can imagine Leliana has reasons.) It's not even entirely clear to whom this tale is being told--presumably the Warden main character, but others are possible.
Leliana has to leave the ending vague so as not to spoil later plot (if you're playing it early) and because the player in a sense influences the outcome. Indeed, in some senses the truest ending comes in Inquisition, and that ending also varies with the player's actions. It's impressive to see a character-driven piece that still allows for player choices, especially considering those player choices are yet to come.
Conclusion: "Leiliana's Song" is an interesting addition in character and lore to the Dragon Age universe.