http://en.wikipedia.org/wiki/Willem_de_Kooning
http://en.wikipedia.org/wiki/Mark_Rothko
http://en.wikipedia.org/wiki/Franz_Kline
It's important to keep in mind that these paintings are not about technical quality. The argument, "my kid could have done that," is not relevant since your kid didn't do it -- you may as well argue you teenager could have written TS Eliot poems since she knows the vocabulary. These paintings are about energy, form and scale, often lack of form or contrasting forms. They also tend to photograph poorly, so try to appreciate them in person.