What makes a drug "powerful?"

My doctor called the MAOI "powerful." What makes a drug powerful? Is it the number of side effects? That really would be a bad indicator if its potency; but a good indicator of its, well, side effect profile. What makes one drug, say, Effexor, more or less powerful than, say, Elavil? MAOIs are often called powerful. Is it because they often work when nothing else works? Is it because they have a very high percentage of success in relieving symptoms (not taking into account side effects). Is it because it affects three neurotransmitters, as opposed to just one or two?