I am just wondering what kind of things could cause power to be lost in general (the opposite of reducing the noise of the data to increase the power of the test).
Could anyone give some summary or ideas how to think of the power issue?
There is an enormous literature on this subject; I'll just give you a quick thumbnail sketch. Let's say you're testing groups' mean differences, as in T-tests. Power will be reduced if...
If you google "power calculator" you'll find some nifty sites that demonstrate the way these factors interact, depending on values you input. And if you download the free Gpower3 program you'll have a nice tool for calculating power and considering the effects of different conditions--for a wide variety of statistical procedures.
Think of power as your ability to recognize the truthfulness of one of two competing data generating processes. It will obviously be easier when:
If the above do not hold, it will be harder to choose the "true" process. I.e., you will lose power.