Rogers holding back Thanos
So the newly released digital download's commentary track spells it out
Thanos was deliberately holding back against each of the Avengers because he loves the sport
But in Steve's case, he wasn't remotely being challenged. That look on his face was supposed to be amusement with a touch of admiration. Then he punches Steve down (just hard enough to knock him out, again on purpose).
They also point out that the gauntlet-grab and punch was slowed down considerably.