I think there is a strong and reasonable argument to be made that nothing about this API is really suitable for beginners. Optimizing for predictability seems like a higher priority than progressive disclosure of complexity.
With all due respect, I would argue that, equivalently, a language with a static type system is also not really suitable for beginners in programming. Does this mean beginners in programming should not start learning programming with Swift? As far as I have seen until now, Swift has been welcoming to beginners and when atomics are added to the standard library I'm fairly certain users with no prior experience in atomics will use them to implement counters, flags and other simple algorithms. Some of these people will get fascinated by them and will continue on their journey and learn more about the intricacies of memory orderings, cache line invalidation, etc. Should the rest have to figure out all these things before they ever implement a counter? 
My argument is that, as I understand it, sequentially consistent ordering offers the strongest guarantees, relieving the user from the burden of having to reason about a bunch of things that in my opinion sit one level lower than most people will care about. The end result is that the final implementation will be correct by default (assumming the algorithm itself is implemented correctly in the first place), and that at worst it will perform slightly worse than an implementation with manually specified orderings. In that regard, and I may be wrong, I consider memory orderings as a tool for optimising an algorithm.
That said, I'm also okay with not having a default if people end up agreeing against -- like it was mentioned upthread, this can simply be added in an extension as needed.
: granted, you might argue that a
relaxed memory ordering typically offers enough guarantees for a counter, but a user would still have to figure this out for themselves, if they cared enough, but a default
seqcst ordering would still be correct.