Mike Atherton has posted a really good tribute to Bob Willis on The Times website - seems it was cancer and had been undergoing treatment through the summer.
I got the Botham's Ashes tape for Xmas in 1994 when I was 15 - watched it hundreds of times, his spell of bowling and Botham's performances just made me love cricket even more (most of the England cricket I'd seen on TV was pretty average being the early 90's, so seeing highlights of England being great was a treat!).
I personally loved his honesty. You can tell from what friends have written that he was a totally different character compared to what we saw on the verdict/debate. However harsh he was, he was always first to praise the players when they performed to their ability.