To be fair:
As far as any normal-mode content is concerned -- that is, anything someone can reasonably queue for in Duty Finder -- I can't recall any "obscure" tells. If someone can see the tell, then they have a good chance of knowing what they need to do.
(I'm setting aside concerns about reaction times.)
Having said that, my "if" there is carrying
a lot of weight, and there are two aspects to this:
- Cast names are meaningful, but the default UI and tutorials makes no effort to highlight that information.
- Visual effects in Endwalker and Dawntrail have emphasized flashiness to the exclusion of all else. These effects look fantastic in a job trailer, where there is a single character going up against a striking dummy, but those same effects are utterly oppressive and blinding when you have a striking dummy that fights back, or 3 or 7 or 23 other players alongside you.
So, in practice, the boss might be casting, "I Will Fail My Right Arm Like a Crazy Person in 5 Seconds," but good luck actually seeing that in the cast bar or in their character model.