"[Downloadable content] and non-EA games purchased on Origin are excluded from the Great Game Guarantee," EA noted in its announcement of the program. "So, this is a guarantee for full game (a.k.a. "base game") digital downloads published by EA for now, including full games on pre-order and most full games on promotional discount."
Don't get clever and think of using it as a rental service; EA says it will remain vigilant of abuse of the refund system. Valve's Steam digital distribution service does not currently support returns.