AbdulAlhazred
Legend
I suppose it would hinge on whether storm hammer is "a power designated as a feature" even though it is specifically listed as a "Cleric Attack 1" and not a "Cleric Feature" (unlike, say divine fortune).
The nut of the question. DS is basing his statement on the idea that since you aren't ACTUALLY given a choice of powers with Warpriest that you don't have the option to pick a power from a different source. Likewise the retraining rules don't allow swapping a class feature (this was true in the PHB version of the rules as well though it had little impact). However when you read the text of the rule it also talks about powers with a level and DS's interpretation definitely has not been accepted by say the charops board people etc, nor has CS issued an FAQ entry clarifying it. He makes a good argument, but it is still a bit of an open question.
As for people claiming that E-classes aren't balanced against earlier classes, they simply haven't PLAYED them together or they wouldn't claim this. Every single report I've read of people playing them side-by-side has stated the balance between old and new is pretty much dead on. Nobody is really certain that an Epic Slayer is going to be exactly as strong as an Epic GW Fighter for instance, but they are well balanced in heroic tier and there's nothing to suggest that will change appreciably later.
The basic upshot is that if you are playing a Martial e-class or Warpriest that MCing may not be as useful as normal and Essentials features don't show up as Hybrid options. Other than that and possibly some issues with Themes (which are still officially DS only anyhow) you have NO compatibility issues.
This is vastly different than 3.0 to 3.5 where virtually all the fundamental combat rules of the game itself changed and classes themselves were rewritten in fundamentally incompatible ways. If you squinted hard you could RUN a 3.0 character in 3.5, they ARE similar games, but you couldn't for instance MC between the two AT ALL and your 3.0 character would have abilities that no longer existed in 3.5 or worked in a very different way and vice versa. 3.0 and 3.5 are 'compatible' the way 1e and Basic are compatible, they're close enough you can 'wing it'. 4e and Essentials are FAR FAR more compatible than that and anyone saying differently obviously hasn't read HotFK or HotFL.