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.
Hollywood Homely: Pilsook is described as a talented, but ugly girl who is first introduced in a bulky, unflattering Mascot costume while wearing a Fat Suit with geeky glasses and Messy Hair to boot. The actress playing her is IU, a pretty, popular singer recently nicknamed as Korea's Little Sister because of her massive popularity.
Episode 6 has Samdong using Jingook's underwear. Jingook is rather upset by this and attempts to force them off of Samdong. It culminates in Jingook pinning Samdong onto a bed and trying to yank off his pants — but they're interrupted by Professor Kang.
Episode 1 also has Jingook admiring Jason's dancing skills from a distance and thinking back to them once he's alone.
Samdong and the lead singer of the Japanese band, having known each other for two days, exchange gifts, do each other favours, and hug twice. The smile the Japanese boy gives Samdong when he realises he's there to cheer him on at his gig can only be described as flirtatious.
Some Anvils Need to Be Dropped: The show has been unusually serious for a Korean Drama in choosing to approach the topic of sexual assault and the prevalence of the casting couch scenario in the idol business, as well as the mud-slinging and bashing the victims often face after reporting their assault.
Values Dissonance: Hyemi calls Jason out on this, telling him that his extra friendly American-style "manners" could mislead naive Korean girls (namely, Pilsook) into thinking he's interested in them.
WTH, Casting Agency?: How most people have reacted about the Idol Singer-filled main cast. It did not help that the it was the female lead's first acting gig and the general consensus is that she initially underperformed in comparison to the more experienced second female lead.