Apart from their technical statement and proof, a usual presentation of theorems is by leading up to them with a definite motivation or intuition, for example putting the results in the wider context of a research programme. This focus gives the author the ability to distinguish between "critical" and "technical" assumptions. By critical assumptions I mean the ones that are crucial to making the proof idea work in the first place. Technical assumptions are then the ones that just happen to be needed to straighten the details out.

For example, from this point of view, a topological space being compact Hausdorff could be said to be a `mere' technical assumption in Gelfand duality, but a critical assumption in Urysohn's lemma.

For another example, an irate referee once taught me not to speak about "mild assumptions", because "under mild assumptions every group is a ring".

But the line between technical and critical assumptions is vague and flexible at best. This question is about when the line moves.

Are there interesting examples of critical assumptions that later (e.g. via a new proof for a generalized setting) turn out to be technical? And, more interestingly, vice versa, are there good examples of technical assumptions that later (e.g. with a different motivation) turn out to be critical?

essentialfor the proof to work. Until you have a proof that shows that a technical assumption is not really necessary, it remains crucial. Once you have such a proof, then that assumption is gone and is equivalent to any other assumption you havenotmade. Of course, there are assumptions that seem to be more natural than others,... – Sándor Kovács Nov 15 '11 at 2:41