the best think about that riff its its ability to be improvised around. i could easyly sit down and bash out loads of different variations of that basic riff, and realy the whole song (the little i can play!). seriously, try playing the intro riff on a 12-string using tremolo, flemenco picking before moving to a Em-G root chord sequence, accompanyed only by percusion. it is amazing how versitile the riff is