Well said. For me it comes down to that golden word, and it's one that people like Dance, Headey, Coster-Waldau, Diana Rigg, Sean Bean and Michelle Fairley all had; gravitas. If you can own the scene just by sitting there and in silence eating an orange then you'll shine. Of the new crop you had people like Jack Gleeson and Richard Madden who had it as well but they're long gone. Conleth Hill, Alfie Allen and Peter Dinklage all have a shit ton of it too but nowadays they have to share screen time with the "main cast" (Jon and Dany) whenever they get a line which is like me trying to recite Shakespeare whilst the majority of the time the camera is focusing on an egg.Aye, it's a real problem that 3 of the main protagonists are played by wooden actors/actresses.
Kit Harrington, Emilia Clarke and Sophie Turner are very attractive people but they're barely even in the same profession as someone like Charles Dance, or even some of their current costars like Lena Headey or Coster-Waldau.
This is pretty much summed up by their work outside of GoT. As you say, Harrington has been awful in his other roles, Turner is dreadful in X-Men and Clarke was similarly shit in Terminator. They just don't have the ability to capture the audience like some of the great actors/actresses that this show has seen.