Why is Gangkhar Puensum the highest *unclimbed* mountain?  It was attempted by four expeditions in 1985 and 1986 after Bhutan opened its mountains for mountaineering in 1983. In 1994, however, climbing mountains higher than 6,000 meters was prohibited out of respect for spiritual beliefs and customs. In 2004, all mountaineering was banned in Bhutan so Gangkhar Puensum will likely remain unclimbed for the foreseeable future.

Source:  http://climbing.about.com/od/mountainclimbing/a/GangkharPuensu.htm

Why is Annapurna 1 the most dangerous mountain?

Not only is Annapurna the least climbed, it has by far the highest fatality rate. A total of 58 climbers have died on Annapurna -- a fatality rate of 40.8%. This is an astonishing statistic, especially bearing in mind that Annapurna, unlike Everest, is not assaulted each year by dozens of teams composed of wealthy but unqualified climbing tourists. Notwithstanding the legions of amateurs on Everest, and its many celebrated climbing disasters, only 192 climbers have died on Everest -- a fatality rate of just 7.5%. Nanga Parbat, which is the third least climbed 8000 meter peak, and which I would have thought the most dangerous -- it has a reputation as the "killer mountain" --, has a fatality rate almost exactly in the middle between Annapurna and Everest -- 23.4% -- significantly lower than Annapurna. The fatality rate on K2 is almost exactly that of Nanga Parbat -- 24.1%.

But WHY?

According to the famed American climber, Ed Viesturs, "Annapurna is all about objective danger, it's all about the glacial architecture. There are these big ice cliffs and seracs, and the question is: are the seracs leaning forward or leaning back? It comes down to that." His account of the Annapurna climb and his earlier failed attempt on Annapurna is chronicled in his new book, Himalayan Quest: No Shortcuts to the Top.

Source:  http://blog.stadum.com/2007/02/annapurna-most-dangerous-mountain.html