For people who don't know who he is to sum it up he was the lead in a band called The Velvet Underground who were a massively influential band in the 60's. Their biggest "hit" was Sweet Jane. But they weren't really influential because they had hits, but because they were incredibly original and quite experimental and had 4 incredible albums. They broke up by the end of the 60's and Lou Reed went solo. He had an awesome album called Transformer but most of his other solo stuff was s*** (imo).
Still, an absolute legend and I think people are just kind of shocked as I don't think anyone knew he was sick/ill/in poor health but hey he was 71.