I agree with RES-can:
Criteria: plural (although 'set of criteria' sounds fine to me)http://www.google.co.uk/search?source=ig&hl=e...
'Criterion' is, I agree, rarely used nowadays, and I don't think I've ever heard it in speech except from a grammar pedant friend of mine. But, if I was writing an official document or an academic report I would never use 'criteria' as a subsitute for 'criterion' (or at least not yet - I am in agreement with RES-can and Merriam Webster that this will change!). I think it would be acceptable in one of those formats to say 'the first criterion is...', 'the last criterion I would like to discuss is'.
HOWEVER, I think most people would tend towards a different noun, rather than say "criterion" - i.e. the condition, the standard, the factor, the decisive question, the principle, the measure etc etc.
That doesn't really answer your question!