Imagine a planet similar in chemical composition to Earth. Could life evolve there for which iron in its elemental form (not necessarily oxides) was toxic? Could animal type lifeforms exist that had somewhat similar respiratory/digestive systems to Earth animals in that they consumed carbon and oxygen, but have a non iron based method of transportation of oxygen. Basically, is it actually feasible to have non-iron based blood (I know Mr. Spock does but I'm not sure how plausible that really is), taking into account that resources are available in a similar proportion to Earths.
Furthermore, if an intelligent species arose, would it have an impassable hump in technological progress to get to a level of technology beyond modern day earth technology without relatively easy access to an abundant metal. That's not to say that iron or steel couldn't be used, but it would be used cautiously and not in everyday items. I realise they could use bronze for blades and other tools, but it is expensive and that aluminium is very abundant and can make a good substitute for iron or steel in many applications, but would you ever really get access to it, if you didn't have the iron and steel to make industrial machinery to extract it? What lines of material technologies might they pursue?