Mark's showing a home video made by his daughter. There are a lot of puns.
EDIT: They briefly explored having boats in Theros, but Mark said to hold off until an artifact set.
EDIT: They tested energy with instead putting charge counters on artifacts and you can take counters from any artifact you control, but that was much harder to track than the current version. Mark says energy will be easy to understand once you start playing. Erik(?) says in contrast to mana, where you want to use up as much as possible in one turn, you will use energy differently.
EDIT: There are fans of fungus tribal in R&D, but they need the right world for it.