Why is the Volcano ash-cloud very dangerous where concerns air-travel?

I have noticed alot of lightening storms and dark dust clouds forming in some of yahoo's videos above the volcano. I would like to hear peoples opinions about the dangers that the clouds poses to the airplanes, communications, weather and to air travellers in general. Although I have some umptions of my own, I believe that it would be useful for people to be properly informed about the professionals and decision-makers reasons as to why our flights are being restricted citing safety reasons over European skies due to the Icelandic Volcano.