.. and, why does it say UNPLUG CHARGER when it's done charging? My other phones have no problem sitting in their dock all day when not in use.
It means unplug the charger from the wall socket or turn it off to save energy once the charging is finished.
Just like to share, that most of the devices like laptops, tablets and phones, are not meant to be wired-charge all the time. In fact keeping it plugged reduce the battery life.
My son, wife and I, all use mackbook pro, we bought 3 machines on the same day. My son's mac is plug all the time, and by now, about year plus later, it won't last without charger for more than 50min. While my wife's and mine, keep pretty much the same life time as we bought it.
Calibrate Your Android Phone, Laptop, Macbook, iPhone Battery Life - The Tech Loft | The Tech Loft
and apple's official battery care guidelines:
Apple Portables: Calibrating your computer's battery for best performance
Apple - Batteries
I also experienced that with our Nokia phones. This time is my wife's, her phone is with shortest battery life among us. She leave it on the charger over night at home, take it to office and charge it there all day long. And even tho it shows 100%, it drops to bare minimum within minutes of usage.
I don't know how that applies to our Galaxy, but I practice the same charging habit as with other devices, for which it seem to work very well.
Check out the links, some of them explain in details the reasons behind it.
Best regards
Sebastian