Vibration (read: increased sound) and slightly reduced airflow, mostly.
To reduce the vibrations, which in addition to increased noise level can cause mechanical harm to the fan bearings and cause it to eventually fail sooner than without the increased vibrations, you should cut the opposite blade off as well to balance the fan, but that'll reduce the airflow even more. That will mean you need to increase the fan speed to keep the cooling to same level as intact fan.
Was it a GPU or CPU heatsink fan, by the way?
By the way... how in the name of all that is holy and sacred did you manage to break just one of the blades anyway? Were you poking it with something while it was spinning, or did you step on it or drop it on something?
