DNA computer is not a computer but a data storage, right? Organic matter is not suitable for a long term storage, especially DNA, it denaturates above 90°C, can be decomposed spontaneously or by ultraviolet radiation, pH change and chemicals and other factors. And who would watch thousands of years of video anyway? I would prefer having the quantum computer instead.

Head transplant – why the whole head? it’s only enough to transplant the brain

moon base? not in this century. Have the astronauts really been on the moon in 1969? who knows… mars mission – not in this century neither.

Clean energy? Already there – what about the nuclear power? cheap, clean and abundant. The renewable energies are not that clean as it appears at the first sight. Photovoltaics, for instance, requires huge amounts of batteries, as result of this lead pollution arises. The solar cells themselves need large amounts of energy to be produced and their production is harmful for the enviroment. Wind turbines hurt birds and wildife.

Space elevator is not possible to construct even with the nanotubes because simple calculations show that the weight and the forces acting on the cable will practically exceed van-der-waals force. One can add to the list perputuum mobile or nuclear fusion aswell =)

Something which is more real and achieveable : holography, laser weapons, robotic assistants, weather control (HAARP)