The watching/giving control of your single player game to a friend sounds awesome. Back before the mortgage, kids, cars and demanding jobs, I used to go to my buddy's house the next town over. Me, him, two high school pals and my brother would all get together once a week and play single player games like RE4 and Mega Man. Play a little bit, pass the controller and shoot the shit. These days, my brother is in college full time, my one friend has a wife and three kids, I have a wife and two kids, the third has a brand new wife and his first child, and the other's whereabouts are unknown. None of us live within 45 minutes of each other now, so the thought of a game like Mega Man 10 (I know, it won't be on the ps4, it's just an example) being played through the PSN, each taking turns with a fundamentally single player game brings back great memories.
Of course, none of this matters if they can't get it right. Full voice chat, zero lag and seamless control are an absolute must.