If McDonald's adopts a composting policy, others are sure to follow. Everything McDonald's creates as waste could potentially be composted, especially if they switched out plastics for biodegradable plastics. According to the EPA, food waste is the single largest waste stream in the US by weight (which is different than the newspaper waste, which is #1 by volume).
In 2007, over 12% of our garbage was made of food scraps, which could easily be composted. When food decomposes without oxygen it produces methane, which is 21 times more damaging to the environment than carbon dioxide. Landfills are the single largest methane producer in the world, accounting for over 34% of all the methane released into the environment (source).