Why did the microwave oven only heat my coffee half as much as expected?

Your microwave creates a standing electromagnetic wave inside itself but it doesn't consume much energy to create this wave, so if the oven is empty it will only consume a small amount of power - a perfect microwave oven would consume no power when empty. Power is only consumed when you put something in the oven that absorbs energy from the standing wave. The oven needs to take energy from the mains to replace the energy absorbed by whatever's inside it.

The rate at which something inside the oven absorbs energy depends on what you put in. For example if you put in a cup of cooking oil it will absorb energy slowly and heat up slowly (I wouldn't try this at home as traces of water in the oil can boil explosively!!).

The 750W rating of the oven doesn't mean it pours 750W into whatever is inside it; it means that's the maximum amount of power it can pour in. The actual power absorbed will typically be less than this and possibly much less.

It would be an interesting experiment to try heating two (or more) mugs of coffee at the same time. I would bet you'll find the total energy absorbed increases as you put more mugs in, up to the 750W limit.

Later:

Prompted by Anna I have done the experiment. I used two identical coffee mugs containing 400g of water each. I first heated just one mug and measured the temperature rise every ten seconds from about 10C up to about 25C - I didn't want to go higher because you have to start worrying about heat loss to the mug and air.

I then replaced the water and heated the two mugs together, spaced as widely apart as possible, and again graphed the temperature as a function of time again up to about 25C. The results? Well my oven is rated at 600W and with one mug I measured the rate of temperature rise to be 481W (plus or minus a few percent). With two mugs the rate of temperature rise was 530W.

Now I'll just post the results up to the arxiv :-)

The 750 watt output on your oven is usually measured using an IEC standard method that requires using one liter of water at 10 C. It is an averaged number, so your oven can actually have a much lower power,(they usually do - I'm just publishing a paper on this) by as much as 20 % or more. Second, as the water load becomes smaller, the efficiency of capture of the microwaves decreases, so the water or coffee may only see 100 or 200 watts of microwaves - a good thing, otherwise small loads would heat much too fast. Also, note that water heats most efficiently at lower temperatures; as the water temperature increases the efficiency decreases because the resonant frequency of the water increases, but the microwave frequency stays the same. These are some of the major reasons. This is all well known microwave oven physics.