Eurogamer's Chris Donlan has penned an editorial on side quests, and what makes them often more compelling than the main plots in RPGs and tangential genres. Here's a snip:
A big part of the answer, of course, is that we're always fighting to save the world, or the galaxy, or the fate of the entire universe. The Reapers are coming, the Covenant are coming, Diablo's coming, the British are coming. We know these arcs so well, and we also know how these things often turn out: the party is gathered and the super-weapon is readied, or maybe there's a last minute suicide mission behind enemy lines. Frequently, it's a bit of everything, but regardless of happens with the detailing, the end result is generally the same: onwards to the heart of the trouble, stand by for a bit of quick monologuing, and then we're out in a blizzard of QTEs.
I don't mind this structure, really. It moves things along pretty swiftly, and it manages to be strangely comforting in its bombastic excess. It's nice to step away from it occasionally, though, and it's nice to venture off-road a little bit. Ah, the side quest!
And so side quests are, amongst other things, a chance to see game designers trying to tell different stories for once, whether they're riffing on 1980s movies, as both Torchlight 2 and Borderlands 2 do in their own distinct ways, or just exploring a different side of heroism. Torchlight 2 offers a lot of scope for small-scale heroics, for example: I know you're saving the world, but can you stop for a second to save my wife, my friend, my husband? It all amounts to the same kind of progression as saving the world, perhaps - and it certainly continues the rich theme of smacking skeletons into dusty fragments - but it somehow feels subtly different. It's more personal, more comprehensible. It's perhaps more heroic?