The Quick start rules explain the basic core mechanics that both Mutants & Masterminds and DC Adventures utilizes (easy since they both use the same roll a D20+ Modifier vs Target Number mechanic). Basic combat and damage is covered, stats for both Comic Book Versions of the current Superboy and Knockout, and a scenario revolving around a Smallville Town Fair.
DC Adventures will be 100% compatible with Mutants & Masterminds 3rd edition, and honestly is usuable with Mutants & Masteminds second edition with no conversion.
Most of the changes from 2nd edition revolve around Attack and Defense being renamed, Skills being consolidated, and some power reorginization and power pricing adjustments, but if you've played the Award Winning M&M 2E than you can pick up and play DC Adventures wihtout even reading the rules.
The Quick Start is Free.
Has characters currently available nowhere else (Until the Heroes and Villains Supplement comes out in December)
Has a scenario/adventure easily adaptable to any characters you have.