The Onion AV Club has put together a detailed article looking at the roots of emo. The article attempts to find the value in the much maligned genre by looking back at the forefathers like Sunny Day Real Estate, Rites of Spring, Jimmy Eat World, Mineral, Pixies, Fugazi and more.
Regarding Fugazi's influence:
What Fugazi harnessed better than any other was the power of restraint. Hardcore had been all about balls-out intensity, but Fugazi's twisty rhythms and patient calculation made the louder, more intense parts of songs that much more powerful.
Diary hovers in a difficult-to-find sweet spot, alternately contemplative and cathartic, an engrossing mix of melody, power, and atmosphere. For so many bands that came after Sunny Day Real Estate--who would essentially break up (for the first time) just months after the album's release--Diary was the blueprint.
Check it out here.