geography and geology
I've followed this thread with interest and some commenters have commented about other volcanic eruptions elsewhere in the world and their apparent lack of effect on air traffic. Their location is one factor; mostly around the pacific rim and many in the southern hemisphere; should one look at prevailing winds, concentration of landmasses, population density and airspace options in these areas one would notice there are far more options for avoidance as well as posing far less of a hazzard for airports (as there are very few in the middle of the pacific, large swathes of south America, northern Canada, etc.). Secondly, the volcano in question is under a glacier and, from what I can so far ascertain, it is this that is creating the levels of ash that are causing problems....molten rock meets water and....(I may be wrong but from what I have read this seems to be a factor).
All very frustrating but personally, would rather be safe than.....
And yes, hopefully this will provide an opportunity to research this further. It hasn't probably been investigated as thoroughly as it should perhaps because of aforementioned relative lack of occurrence in busy airspace.