There's nothing wrong with 2D characters like with Vance or Tolkien I suppose. To be able to have 2D characters and still write well is certainly an accomplishment... These characters serve as archtypes I suppose. Perhaps. Eh. The settings of Vance and Tolkien were "realer" than any others I'm aware of and i wonder if 3D characters take away from the setting...
Perhaps what it is is that relatively 2D characters are such that the reader can flesh out the character by putting themselves into it. And then from that vantage point, a point within the setting/world, they look at the world/setting around them. Whereas with better developed relatively 3D characters the reader may not put themselves into the character as much, they instead look down upon the character and thus their vantage point is farther removed from the setting/world and thus the world doesn't seem as real... although the character may seem realer... Or something along those lines.
