When I think about the UF I still follow, I find they've got a disturbing level of similarities that I hadn't ever noticed. However, it's not exactly a life-changing realization, because it's sort of the cliche, anyway:
Loner MC makes friends, builds a family unit, and carves out a safe-haven/home worth having, while fighting interesting monsters by being clever, on the side. By the time the series diverts from this, I'm already (usually) invested in the characters enough to go along for the ride.
See: first few Anita Blake books before they started being shelved PNR, I mean). Kim Harrison's Hollows series, Seanan McGuire's October Daye books, Cassie Alexander's Eddie Spencer books, Simon R. Green's Nightside series, Jim Butcher's Dresden Files. Anne Rice's well, just about everything. Buffy: the Vampire Slayer.
It's not anything groundbreaking about them that keep me buying the books and turning the pages. It's a familiar, comforting story. You might be alone now, but you're one random vampire attack away from having a place to belong. I think a lot of readers are looking for that message, which is why things like Harry Potter resonate/stand out - it's the same story. A boy who has nobody and nothing finds family, friends, and a home. He's vulnerable, and young, and has magical, amazing adventures, and does the right thing even when it really sucks. For someone who reads to escape, that's the most comforting story out there.