The reason I'm kind of worried about this is that I can see myself using it on a nodejs server to write request handlers that are run with high concurrency.

The thing I like about explicit use about a callback, promise or await is that looking at the code I can see that execution is suspended. I think it's pretty important to see these places, because it's exactly the ones where race conditions may occur in an execution model such as node's (or that of a browser). On a large enough project the implicitness may create more problems than it solves. If for some reason (e.g. a large enough team) any of the functions called from a suspend function becomes suspendable itself, the change passes silently, even though it potentially introduced a bug that only surfaces under high concurrency and therefore in all likelihood won't be caught by tests either and instead occasionally occurs in production - the kind of bug that nobody wants to have to deal with.

I can see the appeal of the implicitness and in many scenarios I think I would use it myself (e.g. in functional code or in small scripts). So I wonder if we could somehow make this an opt-in/opt-out. Perhaps something like using ImplicitSuspend;/using ExplicitSuspend; would be good, because then you can make it per module or even per folder with import.hx.