For a while in my early 20's System of a Down was one of my most listened to bands. They still rank on my have to have on my playlist and I was deeply saddened that my scheduled conflicted with getting to see them play when they were in Toronto for Heavy TO last year as they are awesome live.
I think what I like about System is really the same thing I like any artist that I like for more than one song, or one album, they have a range when it comes to their music. It doesn't all sound the same, there are a variation to the messages that comes across through it. There's something behind it in the first place.
Two examples of what in my opinion are vastly different songs from SOAD are Aerials and Prison Song: