KMFDM has a strong anarchistic political leaning. Its music tends to be nihilistic anti-government (not this government sucks, let's make a new one but government itself sucks), anti-organized religion, cynical about all economic systems and the like. System of a Down is a leftist band, but from the lyrics I've heard it can't be really described as anti-American the way rage can. The biggest distinction between SoaD and RATM is that SoaD has actively avoided trying to become the next media darling political band which is the exact opposite of what RATM did. Oh and SoaD's music videos are far more artistic and traditonal than RATM's which tend to be nothing more than a video-collage-turned-political-soapbox.
The way that SoaD approaches politics is more along the lines of writing political essays for their website rather than trying to turn complex politics into catchy rhymes. And of course, they have a singer that doesn't sound like a crackling just hitting puberty teenage boy (RATM's now former vocalist... can't call him a singer.... Zach De La Rocha has one god awful voice!)
do you know how yoko ono's singing was said to sound like a 'retarded person being beaten'? I think Zach La Rocha's voice sounds like yoko ono being beaten with a retarded person. (sorry, if I have offended.)