I may be wrong about KSH, but I'm certain that if an actor can be 'good' they can also be 'critically acclaimed', i.e. have their skills and body of work praised by many critics.

Sorry I can't reply to you post above so I'm putting my answer here. I was talking about acting, not box office or ratings. This is my personal opinion, based on my limited experience of KSH (e.g. I haven't seen any of his earlier dramas like Kimchi Cheese Smile or Jungle Fish): IMO KSH hasn't displayed the kind of range or skill that some other actors of his generation have. Of course people like Ryu Joon-yeol or Kim Nam-hee have not been anywhere near as successful as KSH when it comes to ratings or box office. And neither of them is a big star with lots of leading roles. But RJY has played scumbags, geeks, nice guys, country bumpkins, drug dealers, cops, etc, and has done so consistently well, whether he's lead or supporting. Kim Nam-hee hasn't even had a lead role, and he's two years older than KSH, but people have praised him for his completely different performances in Sweet Home, Mr Sunshine, On the Verge of Insanity, Mad for Each Other, etc.

In contrast, AFAIK KSH has mostly played people who are quite confident or have many positive characteristics or are cool/heroic. While he has been good in those lead roles and received many awards, and the films/dramas have been extremely successful, I feel (and I stress this is just my opinion) he hasn't really proved himself as an actor yet. Furthermore, the guy in One Ordinary Day is not only NOT supposed to be a particularly strong or nice guy, but he's also someone who loses a lot of control over his life in a very unpleasant way. This isn't something that I've seen KSH do before, whereas guys like RJY, Kim Nam-hee, Ahn Jae-hong, etc have more experience with less heroic characters.

I'm not saying KSH will do badly in the role. On the contrary, I think he is an interesting choice (see my original post right at the top), cos he does have qualities that may contribute something new to the role and perhaps he has been waiting for a role like that to show off what he can really do. And if that does happen, I would be surprised, and very pleasantly so.

Also, can I add that Ryu Joon-yeol starred in Believer and Money, both of which did very well even though they are rather bleak films? Of course Kim Soo-hyun has starred in about half a dozen very successful dramas, compared to RJY's, er, zero (maybe Reply 1988, but I'd say it's Hye-ri who is the star of that show). However, KSH has surprisingly few hit films to his name. The Thieves did well, but he wasn't the lead actor, while Real was a bigger flop than anything RJY has done. This means that the only really successful film that KSH starred in is Secretly Greatly.

Btw KSH is only two years younger than RJY.