Well, pretty much everything.

The exhaust from traditional gasoline-powered mowers is polluting and puts lots of carbon dioxide into the air, contributing to global warming.

“The absolute worst thing you can do is use a gas mower on a typical North-American lawn,” says Lloyd Alter, design and architecture writer for Discovery Communications’ PlanetGreen and TreeHugger blogs. “It still has carbon dioxide and other gases that are harmful. You can’t call anything with gas safe for the environment.” (source)

If using the average engine powered mower (older model), cutting the grass for one hour is equivalent to driving a car for 100 miles! New models have about 30% less emission, but their emission is still significant.