Another point, I wanted to touch on this idea of 'Turing completeness'. I'm not sure it's entirely helpful to the discussion on Ethereum because, even the computer I am writing this message on cannot be classed as 'turing complete' in the true sense of the definition.

I believe your point is related with script inputs being external to the blockchain? If the inputs to a script must come to that script via a transaction, that internalises the inputs - so I must be missing something?