GoT isn't realistic in the way that the cultures of Westeros are "real" or "realistic". Many analogies and parallels to historical cultures are stereotypical at best.
But GoT is (more) realistic (than other TV shows) in it's depiction of:
a) Complex personal motives and agendas. No person is entirely good or entirely bad, and everyone is a hero in their own story. Most shows don't get it, GoT largely does.
b) Costumes and places. Those are not "realistic" in a historic sense, but they are very well-made and of higher visual quality compared to other shows. The rest just seems to recycle props from the last medieval flic it seems, GoT looks more refined. GoT has visual realism that is akin to a very well-made texture map in a completely fictional computer game.
c) Edit: I forgot one: GoT breaks with a few of the usual hollywood stroytelling tropes, making the story feel less formulaic and more realistic. E.g. gratuitous killing of main characters and sudden abandonment of whole long-prepared story-arcs because life happens (King Rob and the Red Wedding).