What were the reasons for the French loss and the Vietnamese win at Dien Bien Phu?

The major reason for the French loss at Dien Bien Phu was
that they picked a really bad place to put their base.  They underestimated what the
Vietminh would be able to do to threaten the base.

The
French placed a large base in the middle of a valley that was surrounded by hills.  They
thought that the Vietnamese would never be able to put artillery and anti-aircraft and
other heavy weapons in those hills.  Therefore, they felt, the base would be a secure
place from which to prevent the Vietminh from taking over the Mekong
valley.

However, the Vietminh managed to haul heavy
artillery up into the hills overlooking the base.  Once they had accomplished this, they
could shell the French at will.  They even prevented the French from dropping supplies
in by parachute.  Eventually, this siege caused the French position to be completely
impossible and they surrendered.