Pretty much everything degrades lithium ion battery performance. Charging, discharging, being fully charged, being too hot, being thought about too much or too little, etc. If it's plugged in, it's probably hot (running in full-power mode) and fully charged... this kills the battery.

If you don't use the battery it will die quicker, you need to move those electrons, try using AC only 40-50% of the time.

Batteries have a certain amount of slow discharge, even when they aren't running anything. People like seeing a full battery all the time, but I don't think it's very good for the battery's life to keep it constantly topped off.