In the Pathfinder tier list, the Skald is a blue tier 3 — in other words, nearly tier 2. The Bard isn't.
What made it deserve this notation? Is there an extremly powerful particular build or option that makes it blue?
At 5th level, a skald is learned in the magic of other spellcasters, and can use his own magic to duplicate those classes' spells. Once per day, a skald can cast any spell on the bard, cleric, or sorcerer/wizard spell list as if it were one of his skald spells known, expending a skald spell slot of the same spell level to cast the desired spell. Casting a spell with spell kenning always has a minimum casting time of 1 full round, regardless of the casting time of the spell.
At level 11 this's 2/day, and at level 17, this's 3/day. This special ability alone solves many, many problems, but combined with the skald's free level 1 feat Scribe Scroll and access to the skill Use Magic Device, the skald becomes capable of supplying solutions to problems that would baffle lesser creatures.
What keeps the skald from tier 2 is that the skald's only solving such problems 1 to 3 times per day (or by using a previously scribed scroll) and the skald can only solve problems using spells full casters have long had access to (although the full casters may not have such spells available when such spells are needed like the skald does).