There are two major things dragging Marco Polo down and that's the racism and the fact that it really didn't need to be seven parts long. The events of this serial happen over a fairly long time period, making each episode almost its own separate little story on the same theme, but unfortunately that theme is ""Let's get to the TARDIS and get out of here!", "Oh no! Marco Polo isn't letting us get to the TARDIS!"" repeated multiple times