Beating a dead horse, but a little amplification.
Running low can cause elevated oil temperatures, and potential oil starvation at high RPM, under aggressive accel/decel/cornering, and/or unusual attitudes (offroading, etc). That said, liquid cooled engines don't usually get oil as hot, so it's less of a concern here.
Overfilling was covered well by Mbrooks.
Ideally? Keep the oil level in the hashed area on the stick. Technically, it should be a quart from bottom to top of the hashed mark, but I've found it to be a little less---so don't blindly dump a quart in. Start with a half, and eek it up to the full line, and you'll know how much to add the next time you hit the bottom of the hashed area.