I'm not going to give a biography of MJ; you can find that on Wikipedia. But I want to make a few points of my own, and throw my own twopence in.
Michael Jackson was perhaps the greatest force in the entertainment industry; his revolutionary music videos, dance routines and music itself sculpted the way music is made today. I find it hard to think of a genre (includind Hip-Hop, Classical and Country) that hasn't been touched someway by MJs genius.

Al Sharpton perhaps said it best when he said that there was nothing strange about Michael, but that, what he had to deal with was strange. However, Michael was strange, very strange. He was the antithesis of normal. He was extraordinary in its original sense. If he was normal, instead of strange, he would never have created Thriller, Bad, Dangerous, Invincible, Blood on the Dancefloor, or Off the Wall. He would have never have mastered the Jennarow to the extent that it became renamed the 'Moonwalk'. Millions of children in 3rd World countries would have died due to the lack of money his philantrophy provided. The Beatles' catalogue would have probably been bought by some business tycoon and made too expensive for today's youth to become inspired by. Wesley Snipes probably wouldn't have a movie career. Two kingdoms may have never merged (Jackson & Presley...good one eh?). Michael was the strangest person to ever grace the music industry..but never, ever, in a bad way. I think, if any of us were him, we would probably have gone down a similar path of self-induced isolation. Any of us, famous or not, can only imagine how difficult it must have been to have attempted to enjoy some of the subtle pleasures in life while being mobbed by hundreds of thousands of screaming worshippers everyday.
I doubt whether Michael even realised how famous he actually became. There is room in the imagination to suspect that while Michael played along with his perceived notion of how popular he might have been, he never actually realised just how immense he really was.

Th world has it's U2s, it's Guns n Roses (pre-meltdown), it's Nirvanas, it's Beatles, it's Metallicas...but none of them measure to what Michael actually was or did. Each of these groups of performers, I'd suspect would say that his music wasn't their 'cup of tea' but, I'd find it hard to believe that they don't have a favorite song of his, or that they don't know the entire lyrics to one of his tracks.
There is something to be said when an entire industry stands still upon hearing the news of the demise of the keystone in it's arch. Vegas may have turned down the lights for Sinatra; the White House may have hung it's flag at half-mast for Ted Kennedy; the Apollo may have filled for James Brown. But the world, for a moment, stopped rotating for Michael Jackson. The current controversy surrounding the circumstances of his death and the allegations regarding intra-familial disputes over his estate do nothing but cloud the fact that the world is emptier without it's King of Pop.
One thing I would like to add restates something that Madonna said and while her music doesn't resonate greatly with me her; point about MJ was apt. For 15 years the world saw Jackson as a freak and for all the wrong reasons. As soon as some smoke was blown in his direction; he was seen as deviant and abhorrent. When the smoke cleared; he was still painted with that same brush. Now that he's gone, the likes of The Sun newspaper(in the UK) and other such rags fawn at the mention of Jackson. I'd just like to say, shame on you. you were the ones that labelled him a monster. You were the ones that sold papers based on false accusations regarding his sexuality, behaviour, misfortune and curiosity. You were the ones that did to him what you did to Lady Diana and others before him. To have the audacity to write those horrific articles in the past, and then to compile and release 'Tribute Issues' after he is dead is sickening.
The world as I said, is emptier without Michael Jackson. Never will we hear songs like Billie Jean or Thriller again. There are artists with promise, talent and ability to create music that is reminiscent, but not equal. Even Michael, if he was alive, may not have been able to create music as influential or as artistically renovating as he did. Additionally regrettable is the fact that MJ never had the chance convey a true memoir of his life's works. The creative roadmap which led him to manifest his art has died with him. His secrets buried forever. For this, I feel cheated. Not by him, or by the media, or by a Doctor being blamed by some for MJ's death...just cheated. 'The Gospel according to the King of Pop' himself would have been a great read. I know this because to have been fortunate enough to have seen the latter half of these events take place, in his music, short-films, stage perfomances etc. It was amazing and something that I feel glad to have felt a part of.
