These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Ear Worm: The opening, complete with gorgeous animation using the currency from several different countries.
Genius Bonus: As the story develops, people familiar with economics will see the many aspects of the show which are metaphors for real life economic terms and issues, one of the most prominent being the idea of "opportunity cost" which Kimimaro and Mikuni are eventually shown to represent opposite sides of.
The intro has a lot of equations used in finance.
C is also a metaphor for the recent financial crisis/stock market crisis in general and the chain effect which it has on the world economy.
"Msyu is yours": In-show, the announcer declares that an asset "is yours" if you purchase some of its stock during one of its battles. Among fans it's a reference to Msyu's status as Ms. Fanservice.
"A-A-A-A-A-ANGEL!": from episode 6, where Kimimaro's opponent Kou Sennoza wields an angelic asset that uses a Flation named Angel, which drops lightning bolts on the target. The meme comes from a moment where Sennoza sets up several Angels at once, causing the Deal announcer's voice to stutter.
Only the Author Can Save Them Now: Mikuni was the only Entre with a Black Card. He could do whatever he wanted with the Rotary Press, and the heroes had no means to fight against him. When Kimimaro goes up against him, why, look, the mysterious higher-ups had issued Kimimaro a Black Card, giving him the right to deal against Mikuni for control of Japan's futures.
Uncanny Valley: An interesting and probably intentional example: Masakaki is animated by a different method than the other characters, which does even more to make him look subtly unnatural than his bizarre coloration does.
The Woobie: Hanabi's condition gets worse as the story goes on, resulting in full-on manic depression with bouts of insanity even though she was a cheerful, bright girl not so long ago (as in episodes, in-universe is different). She really needs a hug.