Side Note: This made me wonder about other Primarch's Morty wouldn't like. Or ones he would like.
Until we meet the other Primarchs, the only indicator we have right now is the Emperor himself. That said:
"When they told me you had become a farmer, I was surprised,"
He certainly expected something very different. In a way we did break his expectations and this is clearly a much different path compared to what the other Primarchs did.
Magnus might come the closest, but only because his purpose always lied in the arcane and the Warp instead of ruling directly.
"It seemed such a waste of your talents."
Seemed being the key word. The Emperor is clearly admitting that he himself didn't quite know what to make of it until he saw it with his own eyes or thought it over. It at least shows that the Emperor had his own expectations, but is willing to discard those if he is suitably impressed. (I think)
the Emperor says, looking you over from head to toe with an artisan's pride in his eyes,
Could just be Mortarion's misinterpretation, but this is not the "you haven't met my expectations" look, but rather "I'm actually impressed with what I created and what it became." There are good and bad interpretations of this section.
You come to a halt on just the other side of your gate, feeling the subtle barrier of the threshold that keeps the daemon from stepping closer. This land is yours, wrought and claimed by your own two hands, and that matters to such beings more than it might to mere men.
This is certainly interesting. It's either proof that Mortarion consciously uses Psyker powers or that there's a certain weight to territory, actions and connections that get enhanced by his own nature.
It does show that the Emperor respects his territory and home. He didn't invite himself in, but kept himself on the border of what is physically and metaphysically Mortarion's land. Not just a planet of a Primarch's birth, but recognition of his home.
I came here to ask you to join me – to fulfil the purpose for which you were made, and repeat the triumph of Barbarus across the stars."
Ask and triumph being the key words this time. Could just be the Emperor buttering him up, but it shows a distinct respect to Mortarion. "What you achieved is worthy of praise and you proved yourself as one of my sons," "I see that you have a life of your own, yet I ask you to follow me in my own task."
It's certainly a very respectful showing so far.
The Emperor is a matured man and experienced leader, but even he needed a moment to appreciate Mortarion's decision (that's what I'm getting from it at least). His prior interactions with Primarchs colouring his expectations.
As for the other Primarchs? Most will have the same expectations, but will be far less capable of adjusting them as the Emperor did. I don't expect us to get along with most of them from the start. Probably not even half.