The best rhetorical trick Eggers uses is exactly the one you pointed to: the conversations with imaginary interlocutors. They're not just arguments with strangers, though. All through the book, Will is carrying on one conversation with Hand that's real, and another one that's only taking place in his own head. And there's a stretch—a powerful one, actually—where Will talks to God, and another where Will talks with Jack, although Jack doesn't say anything at all in return.
As a device this may sound kind of obvious, but off the top of my head I can't think of another writer who uses it as effectively the way Eggers does. (He did something similar in the middle of A Heartbreaking Work of Staggering Genius, where he made himself the subject of an imaginary interview.) It gets you inside Will's head without having him just soliloquize at length, and hearing the words Will puts in everyone else's mouths obviously tells us something important about Will (even as it shapes our idea of the other characters). It sounds like a gimmicky technique, but is it? We all have these kinds of conversations in our heads. Eggers does it all very deftly, and it never felt artificial.
I also like the way he sprinkles pop-culture references through the book. Clearly, the idea of using pop culture as a touchstone is hardly some great innovation, and a lot of writers who rely on it are just trying to lend their books a patina of hipness or cultural currency. But Eggers uses pop culture in a way that feels totally real. Take the scene where Will is driving at high speed through the narrow alleys of Marrakesh, and he just says, "I am Ronin," referring, of course, to the De Niro movie Ronin, in which there are all these incredible car chases. Now, if you've never heard of Ronin, the reference doesn't mean anything. But I have no doubt that if I had been driving a car through the narrow alleys of Marrakesh, I too would have thought to myself "Ronin." (I remember in AHWOSG, Eggers made a passing reference to "Westerberg hair," and I thought, "Exactly right.") There actually aren't that many of these moments in the book, but I think they really capture the way pop culture shapes not just our everyday vocabulary, but our everyday experience, and not in a bad or insidious way, either. It's not that we've been colonized by pop culture (any more than people who used to think constantly of biblical references were colonized by the Bible). It's just part of who we are.
So that's the stuff I like. What I didn't like, really, was all the stuff about money and charity and how guilty we all are. It's fair to say that Eggers is writing about something real. When you walk down the street in New York and you pass a homeless guy, every time you do ask yourself, "Should I give him any money? How much should I give him?" and so on. That's real, and if Eggers really feels so guilty about having made lots of money that he wants to give it all away, more power to him. The problem is that you can't in any sense leap from that initial impulse of guilt to the conclusion that if we just gave our money away things would be much better for everyone else.
That leap, of course, is exactly the one that Peter Singer made in the New York Times Magazine article you mentioned, and it's sort of the leap that Will seems to be making in You Shall Know Our Velocity (although maybe not explicitly). I don't want to get too bogged down in the details here, but that whole premise depends on a deeply confused idea of where wealth actually comes from. It assumes that there is this giant pile of wealth that's just sitting there being controlled by rich Westerners, but which we could just divvy up differently if we had the will. But of course this isn't true. All that wealth only exists because there all these millions of people working incredibly hard to earn it, and they're only able to do so because there all these millions of people who are out there spending money and millions of others who are out there investing it. And they're only doing so, ultimately, because they believe all that work is going to improve their lives. In the simplest terms, if you take away that incentive—by insisting that you should just give your money away—you also eventually take away the wealth that all the work creates.
But in a way this is almost beside the point, since You Shall Know is a book that's almost defiantly not about economics. After all, Will feels like he just lucked into his money (one wonders how he'd feel about going to work every day, then going out at night to give away all that he just earned). And while it's possible that the Senegalese and Moroccans Will gives his money to will put that cash to productive use, he doesn't seem to care one way or the other, because he's not thinking that seriously about the economics of global poverty. When you say you want more context for Will's political perspective, I guess I'd say that he doesn't really have a political perspective, other than a vague "Mean People Suck and It's Bad that People Are Poor" sensibility. I was frustrated by this, too, but I have to say I wasn't surprised. One of the striking things about AHWOSG was how fundamentally disengaged the book seemed from political and social questions. You Shall Know's attitude toward money seems, in an important sense, childlike, which has its charms but also its limitations.