Bang Goes the Theory actually covered this in the show today. For those who don't know what Bang Goes the Theory is, it's basically a science/entertainment/educational show on the BBC, and it's very good. They explained why the airspace was closed on this occasion when other volcanic eruptions have had very little impact on air travel in comparison. Basically this type of volcano produces smaller and coarser ash, because it's smaller it travels further in the air and rises higher, and because it's coarser it causes more damage to aircraft and jet engines.
It will probably be on youtube at some point for those of you who can't access the iPlayer, but it's well worth watching.