Several times during seminars I heard the following exchange, which sounds like a joke but actually isn't:
Audience member: Why is X true?
Speaker: Oh, it's obvious.
Audience member: OK, thanks! [sits down satisfied]
Especially in mathematics, it is often helpful to let the Reader know what level of difficulty and/or complexity each step is. The approach you (or at least I) would take differs significantly depending on this.
If a step is supposed to be easy and it's not quite my field, I'd take note that this sort of transition is standard in these parts and move on. Conversely, if the step is supposed to be easy and it is my field and it doesn't seem easy at all, then it's a strong indication that either the author is wrong (rare) or that my understanding is insufficient and I should think about this transition until it seems easy (not just until I find any way to justify it).
If a step is supposed to be moderate in difficulty, I might or might not work out the details myslef as an exercise, but I will also be aware that some amount of work goes into making it possible - helpful when evaluating if an argument is plausible and also in identifying where the content of the paper really is (especially in papers that are heavy on definitions, it's often a nontrivial task to figure out which transitions are easy but unfamiliar and which are responsible for the actual progress).
Finally, if a step is said to be difficult then there obviously should be a reference or a proof. If it's a proof, then more likely than not this is the point where the progress is being made in the paper, so if I'm reading the paper this is the part I would study in the most detail. If it's a reference, I would make a mental note that it's a potentially strong tool to keep in my arsenal - also, I would know better than to attempt to reproduce the result myself.
Note that it's not always all that easy to determine which is which without the Author explicitly making a judgement. The short phrase "By Thm. C in  we have X" could expand into either of "It is easy to see that X (see Thm C in  for details)" and "Because of the deep theorem of Smith (Thm C in ) we have X".
Having said all of the above, I want to add that personally, I dislike the phrase "It is easy to see". I understand the sentiment, but if the paper is read by anyone other than the experts in the field (maybe undergrads or experts in another field) then chances are it's not going to be easy to see to all the Readers, and the Readers who don't find it easy might feel bad about it in one way or another. If I'm already taking the time to explain myself for not explaining a transition I try to give some more details: either a super-short sketch of a proof, or a phrase like "It follows by an application of standard techniques that ..." or "A simple but mundane computation shows that...", etc.