"Bitter Work" is the rough English translation of the Chinese words for martial arts "Kung Fu." Given that Aang and Zuko both learn a different form of kung fu, the episode title is rather appropriate.
Iroh says firebenders create lightning by separating their negative xi and positive xi. In nature, lightning is created as a consequence of negative charges and positive charges within a cloud separating from each other.
Avatar Extras says that Sokka getting stuck in a crack in the ground was inspired by Tom Hanks in The Money Pit (1986).
This is another episode showing the parallels of Aang and Zuko.
The "sea of xi" that Iroh mentions is a real concept in ancient medicine. They believe there are multiple seas of xi, or dantians. The stomach is called the lower dantian.