I believe it is considered a prized coffee because it's in a very special region of Jamaica, a place not known to grow much coffee at all. It costs so much because the price is also based off on supply and demand. Since there is so little of it grown at a time, the demand for it is consistently high.
Have I actually tried Jamaica Blue Mountain coffee to validate the exquisite taste of this? No. However, according to Espresso Coffee Guide, it is typically a washed-process coffee. It is "silky smooth, full bodied," and with a refined taste of sweetness and richness. People also often say it tastes slightly chocolatey.
Like many prized coffees, like Kona coffee and Kopi Luwak for example, many people will scam others into thinking they're buying the real thing–and yet, they will sell something else off as the product. The same happens with Jamaica Blue Mountain Coffee. In order to combat this, the Jamaica coffee board ensures an official seal on is placed on the bags of true Jamaica Blue Mountain coffee.
When buying the coffee, bear this in mind, and look for the seal on your next purchase.