The Aztecs is the first Doctor Who historical adventure that exists in a complete form, with the earlier Marco Polo existing only as stills and audio. Like every episode in the first season, it would contain the germs of many ideas that we are still seeing played out fifty years later. And that is despite the fact that the the shooting budget and production values are close to ridiculous by modern standards.
When I started watching classic Doctor Who, I was expecting to have a condescending attitude, viewing it mostly for historical interest. But "The Aztecs", like almost all of the first season, is a great story in and of itself.
Two of the major things I was expecting in a television show from 1963 were racism or sexism, of varying degrees of explicitness. I found this story to be surprisingly free of either. The Aztecs are portrayed respectfully, as a society that was working under the conditions it had evolved under. Although there is one scenery chewing Aztec villain, that is probably out of dramatic need rather than cultural stereotyping. As far as sexism goes, Barbara Wright, instead of being a follower or victim, boldly sets about trying to reform Aztec society, with the goal of making them more resistant when the Spanish come. Both the character and the actress playing her come across as a strong, intelligent woman.
This is also the first Doctor Who episode that deals with the issue, still under discussion about the morality and possibility of interfering with history.
"You can't rewrite history, not one line!"
The Doctor tells Barbara, trying to dissuade her from reforming and saving the Aztecs. It is unclear whether that was true at the time, or whether The Doctor didn't understand how time worked, or whether he was intentionally lying to Barbara. Whatever the answer to the question was, this was the first time that this question would be asked in Doctor Who.