One specific feature I really like is being able to stub out all instances of a class. A lot of times I do something like the following with RSpec mocks:
stub_car = mock(Car)
stub_car.stub!(:speed).and_return(100)
Car.stub!(:new).and_return(stub_car)
with Mocha that becomes:
Car.any_instance.stubs(:speed).returns(100)
I find the Mocha version clearer and more explicit.