Narrative is fundamental: stories communicating need are the strongest pull, especially if they're about jobs, family and money.
Reciprocation will sway the balance: mentioning an intention to pay back the favor scores big with Redditors.
Status is important: Redditors with higher status overall (higher karma) or higher status within the subcommunity (previous posts) are more likely to get pizza.
Politeness doesn't matter: Redditors don't care much for manners in these posts before they give pizza—but they do appreciate people saying thanks after the event