WARNING:
HEATING WATER IN MICROWAVE CAN BE DANGEROUS.

There are many people who use microwaves that have never really read the instruction manual. If you were to take the time to read the instructions you may be surprised to learn about the danger of heating plain water in it. Here is an example of one Microwave Operations Instructions manual:

Under the heading "Liquids" it states:

Doctors at hospitals say that it is a common occurrence that people have been badly burned due to the experience of heating water in the microwave.

If water is heated in the microwave, something should be placed in the cup to diffuse the energy such as a wooden stirrer stick/spoon, tea bag, etc. It is however a much safer choice to boil water in a kettle.