The use of words such as "clearly", "obviously" etc. in a technical paper
Seconding posdef's appraisal, but being a little more blunt: if one is in a position to get away with bullying or intimidating people by implying that it's their problem if one has not explained well enough ... well, I'd say it's still a jerk-y thing to do. If one is in a lower-status position, such words will often be red flags.

Or, coming to functionality versus rhetoric versus "formal proof": at best these words are functionless filler. That is, saying something is clear is not what makes it clear: if it is clear after these words, it was clear before. Conceivably a thing is clear _once_noted_, and thus deserves "Observe that...". But this, too, can be abused if used outside situations where one is noting that something is "a-fortiori" true, that is, is weaker than what the argument has already demonstrated... but presumably suffices for the issues at hand.