As cliche as this song is, I have to admit I had never heard it until this weekend when my band learned and played it at Topper's. After my first experience with it, I understand why people always scream for it and why it's one of the most overplayed songs ever.
This song is a great example of how powerful simplicity can be. It's The descending bass line created by the G-D6-Em is strong, and then the next three chords, F-C-D, provide a sharp, unexepected cadence that brings us back to the dominant.
The progression is made stronger by the tempo - the slow, hard rock sound makes each chord felt solidly, as well as the fact that the only notes played besides the root, third and fifth are sung or are played during a solo.
The second part of the song, which uses the progression G-Bb-C, is basically as powerful as the first, though it only has plagal cadences, which I'm sure Lynyrd Skynyrd considered in composing the piece. This progression gets pounded into the listener's head in a couple of ways. First, it's repeated over and over again (amazing use of repetition to strengthen the tonal center), and then, after a slowed section, the three chords are put to a march setting that reminds the listener of a defiant army of rock marching on the capitol to take down the man.
In the end, I'm incredibly happy the band sold out to please the masses - I wasn't pleased at first when I heard we were going to work it out, but the effect was amazing, thanks to brilliant compositional techniques employed by intelligent artists.