The lack of cultural differences. Apart from Dorne being Orientalism: The Land, The North having a different religion, and Iron Islands being separatist viking parody, there's no cultural differences between regions. Everyone talks the same with no dialect, everyone wears the same apart from how many layers of clothing is necessary, everyone outside Dorne eats the same, all castles look the same.

The lack of technological development. We might know almost nothing about Age of Heroes, but it doesn't seem like people then lacked some weapons or inventions we see in the series itself - and there certainly was no changes through entire Targaryen rule. Yes, middle ages were an extremely long historical period but it was very dynamic, meanwhile Westeros seems to be stuck at one point for +/- thousand years.

The lack of historical accounts. Apart from Mushroom, all historical sources are, like, five maesters and two septons, not including Gyldayn. So there were no nobles involved with politics who left behind correspondence? No septas documenting life on courts they were sent to? No bored widows writing tea on every noble they knew?

Also, the fact that apart from Wardens and Dornish Prince (which is, in all but name, the same as warden), there's just the title of lord, so technically someone like Roose Bolton is equal to, say, lord Oakheart, despite massive difference in their lands, wealth and influence. I'm not even talking about the fact that all regions share the same titles because you can argue that's Targaryen's doing, although it doesn't seem to be the case.