I'm sorry if this is a stupid question, I'm not a bitcoin user and may lack important information. My understanding was that each bitcoin user has a wallet on his harddrive or wherever, where he stores his bitcoins with the transaction history proving that they are his. So the system is pretty decentralized. Then why is it such a problem that Mt. Gox disappears? Don't the people losing money have it in their own wallets, if so why?
Edit to add: In case the question was unclear, I was asking for why people lost money with Mt. Gox - not about the wider ramifications like potentially lost trust in *coin etc.