As far as I can see, the feats from Essentials are way more powerful than their older counterparts. On the other hand, I see the Essential classes as weaker versions of the older ones (specially Fighter/Knight).
So, are those two supposed to be combined? They don't seem to be meant to be used together. I believe the powers shown in the new books are meant to be used since not even the classes from Essentials can use them, mostly, but the feats, I'm not so sure.
If you create a Knight with the Essentials feats, it might do fine, but a Fighter with the same feats might get a little overpowered, I believe.
Is there any official release about the matter?