The PHB1 human gets a third at-will. Obviously this does not work with classes that don't get at-wills (like the Essentials Knight).
Exactly. The at will stances are clearly analogous with the current fighter's at will powers.
But not interchangeable as it currently stands.
The stances are 'Utility' powers, which the human feature does not permit, as it only allows Level 1 At-Will Attack Powers.
So the PHB human does not work with the essentials fighter, and nor does 'Just take a PHB at-will' really work given the essentials fighter doesn't have access to that. That meaning, the Essentials fighter has to be self-contained, otherwise the Essentials line does not work. The Essentials human must be different in some way to accomodate the lack of level 1 attack at-will powers.
So the PHB human does not work with the essentials fighter, and nor does 'Just take a PHB at-will' really work given the essentials fighter doesn't have access to that.
I think they did something similar with PHB3 when they introduced psionics, where they had (I believe) a sidebar explaining how it interacts with the human bonus at-will.
Humans get a free at-will 1st level attack power as a psionic class, just like every other class.