Humor!
References to other games. The first time I heard "It's not plugged in yet!" in MB I teared up from laughing so hard. I don't think it's a direct reference between MB and TAF, but it still killed me. BOP having PinBot in there is great. I'm sure there are more, but I'd love to see more of these. It's tough with licensed themes I guess, but there's no reason GB couldn't have had Ernie read a few words from an AC/DC or Metallica song in an appropriate place. Or have Burt Ward quote Star Trek randomly in BM66.