Similarities
Similarities between the leadup to the Challenger disaster & the current flying through ash situation I think lie in the element of powerful political, management, public & economic forces wanting to get 'the bird in the air' versus the quiet voice of those involved with the engineering side saying its not wise.
In the case of the shuttle, engineers were warning 'do not launch, safety cannot be guaranteed under these conditions' yet the concerns were ignored. In this current ash debate, powerful voices from the airline industry and politics monopolize the discussion, particularily in the media - whereas those who have expert, detailled knowledge of airplane engines and ash, namely the engineers who design and manufacture jet engines, are less heard (their message: ' ash & jet engines don't mix'). Yet, it appears the public & those in authority to make decisions are hardly hearing the voice of the true experts on the subject during this crisis.
Uncharted territory...
I'm hoping that existing protocols, wind, luck & maintenance regimes will help avoid any potential catastrophic outcomes. Just imagine what an ash-related accident would do to the industry (!)...