Being a travel bug, I have already made my own list of places to see before I reach 60 or before that place totally disappear from the face of the earth, which ever comes first... Kidding aside, these places are not going to disappear but may be closed for public due to erosion, wear and tear condition, pollution, development in infrastructure, exploitation of the place by not providing sanctioned tourist guidelines to follow when visiting these places and neglect from humanity. These places are taken from Frommers' 500 Places To See Before They Disappear.
1. Taj Mahal
2. Lord Howe Island
3. Pyramids of Giza
4. Gu Gong, China
5. Little Green Street in England
6. Hill of Tara, Ireland
These are just few in the list. To know more, you have the get the book ;)