HN/FOB fascinated me from the first time I saw it. Aside from the chameleon arch, which I mentioned a few days ago, the episode centered around the very human life of John Smith and his struggle to cope with finding out that his entire life was an invention. Martha spent her entire time trying to hold everything together while living in social and economic conditions almost as alien to her as any distant planet she'd visited. I remember that on first viewing, I cried my eyes out at John's fate. And the episode did not end well, revealing the darkness and cruelty of this Doctor, both in his punishment of the Family and his subsequent treatment of Joan (which, admittedly, was born out of his inability to understand her, but was still cruel).
But beyond that, there's just so much to love. The characters are well-drawn, from stern Headmaster Rocastle and efficient Matron Redfern, down to the bullies Baines and Hutchinson - one suave and cunning while the other is more of a thug - to the timid but brilliant Latimer, to Martha's only friend Jenny. They aren't just story-dressing, filling out a school that would otherwise feel empty: they have their roles to play in the story as well as their own backgrounds, conflicts, and goals. Joan, of course, gets the most character development, growing from a grieving widow who thinks her heart's hardened by her husband's death into John's love who has the strength to guide him toward the right decision.
And then there's the Family. Single-minded about their goal of immortality and fond of blood and killing, they however are not the typical "destroy everything just for fun" villain that's common in Doctor Who. After a show of force, they stated many times that they would leave peacefully if they got what they came for; they only started attacking the school and shelling the village when John escaped. There was a horde of the typical DW slow shambling monsters, but at least this time, there was an explanation: the Family couldn't risk themselves by fighting directly, so they created an animated army.
The performances were also brilliant. Rebekah Staton and Harry Lloyd were especially amazing, playing first their normal human characters Jenny and Baines, and then their psychotic Family counterparts. We didn't get to see many scenes of Lor Stanton playing Lucy Cartwright, but she was terrifying. And of course the Doctor himself. After nearly two hours of watching John Smith, the Doctor was alien and disturbing.
I might have fallen in love with Doctor Who long before, in the campy episodes of the Ninth Doctor, but this was the story that showed me what DW could really be.