One of the main criticisms of IQ tests (in the US) is that they are culturally biased in favour of white, educated, middle-class americans. There have been a few studies that have shown that if the tests are phrased in a way that black working class people are more familiar with they will score higher than white, middle class people taking the same test. Another criticism is that cognitive psychologists are divided on what intelligence actually is, so it's difficult to accurately quantify/measure something when you don't have a working definition of what it is you are trying to test.