The thing to know about tea and coffee is NEVER BOIL THE LEAVES OR GROUNDS.  This brings out bitter tannins.  Use the microwave to heat the water only.  If you are reheating coffee use a short time period till you find out what works best.  You can't blame your oven if you burn the cake, why do you blame the microwave?  I reheat my coffee all the time without loss of quality, and it has sugar and cream in it.

to heat it up.

My mom used to make her tea in the microwave, until her friend told her it caused toxins to come out of the tea. Not sure about coffee, but I always dumped mine, and just got a new cup.

If I'm busy at work, and haven't finished my coffee, and it's cold, I'll reheat it in the microwave. At home I would never do this. I would just dump it and pour another cup from the pot. But any coffee that's been sitting around too long gets a bitter taste and at that point you just need to make a fresh pot.

I know a few people who reheat coffee in the microwave, but it always tastes unpleasantly acidic to me.