I've been playing DW by skype one-on-one with my girl. We found that (as with any roleplaying game), the conversation between players - in this case just the two of us - is the most important thing to establishing the story. For us, Bonds were exactly as Sage said, a way to tie her character (and important NPCs) to the situation. It felt very Burning Wheel actually.
I suggested that each bond be with a different character 'type'. So after a good deal of 'conversation' during chargen, we squeezed as much backstory as we could out of her move choices and list decisions. Always following each choice with Why?
By asking lots of questions, her ranger ended up with 4 unique and interesting bonds, or nuclei for bangs in play. We were pumped to dive into the situation we had created. There were so many threads to explore!
Her ranger was holed up with her vicious hawk companion in a rambling town on the fringe. Folks had been fleeing from the overcrowded, pestilence ridden city looking for a more simple, healthy life away from the corruption of the city sprawl. Unfortunately, monsters had been encroaching from the mountains, seeking new territories as an unknown menace had been displacing them.
She had guided a wounded racist human warrior once back from an old dungeon complex in the forest and he owed her for it (though he also totally lusts after her)
There is a herbalist / shaman in town that she is friends with due to their nature connection. She trades herbs and potions and pelts. Unfortunately, the herbalist has had her 'lifestyle' insulted by the fighter and they hate each other.
Their is a bard who has fled the city due to a 'misunderstanding' with a noble's daughter. The ranger has decided to teach him wilderness craft as long as he helps her 'fit in' better in the city as she seeks to exact her revenge on the wizard who enslaved her kin.
A dark wizard with shadowy ties to the throne has been enslaving rural folk to work his dungeons under his tower deep in the heart of the stinking city. She hates him with all her heart.
These NPC's appear as needed, it was some great fun writing them up too. They don't act so much as retainers, as 'contacts' (as in BW circles). As the moves (player or DM) integrate with the fiction, the characters enter scenes and leave them too. Haven't found an issue with playing one-on-one. The classes, moves and bonds set up a very interesting and tension filled character.
Can't wait to keep playing! I thoroughly recommend one-on-one :)