This is perhaps my first ever post on this forum (aside from posing Tom a question) but at this point I feel like I can't NOT post.
Like others have said, the level of genuine sadness I felt on reading this news was illogical, even completely inexplicable, for someone that I have never actually met. I'm not a terribly emotional person but this really got to me for some reason. It really feels as though I have lost a dear friend.
And yes, I suppose that really was why many of us feel that Elisabeth Sladen was the greatest companion of all. Watching her, it was as if there was no barrier between Lis, her character, and the relationships and experiences she shared with those on screen with her. It all felt so natural, lacking that element of performance that causes you to sometimes perceive characters as more abstraction than reality. To me, she was a real, three dimensional, strong woman. And watching the end of Hand of Fear was sad enough, but it was all somehow remedied by seeing her appear in The Five Doctors, where clearly the Doctor had "remembered" her, and then later on in the new series/SJA where we knew she lived on as a strong and spirited woman. It was so cool to know that a new generation of children could grow up to love and be inspired by Elisabeth Sladen's character. I can only imagine that she kept her illness secret from the public because she really was committed to staying positive for herself and her fans until the end.