Judging from the other comments on this thread, we have quite a diversity of opinions about what makes great rock and roll, if not what makes great music in general. Personally, I like a bit of just about everything but disco and its variants. When I set my mp3's playing randomly, I can hear Count Basie, The Rolling Stones, Anonymous 4, Pink Floyd, Beethoven, Ella Fitzgerald, The Cars, and the The Sons of the Pioneers in succession. Having ripped every disk I have (vinyl and CD), I just love having all this stuff right here in one place where I can just have my player run for days on end without repeating a turn. Isn't technology wonderful?
Sure is. I've spent most of the past year turning my massive record collection into MP3s. I take them on the road with me with my MP3 player but mostly I just listen to them at random on the computer. I have nearly 15,000 tracks and I am discovering and re-discovering music from classical to blues to vintage rock. It's a lot of fun. I've even grabbed some of those 99 cent songs off the web.
BTW, I have the Symphonic Pink Floyd. That's been out for some years now. Good stuff.