The first story to address the changing of history, The Aztecs is a more interesting historical story than Marco Polo, and shows writer John Lucarotti begin to explore the full creative potential of his time-travelling time.
At the heart of The Aztecs is Barbara, who is immediately worshipped as the incarnation of a God by the local Aztec people. The ensuing performance by Jacqueline Hill is a hi-light of the series so far. In The Aztecs we see an actress coming into her own with a character. Barbara is by far the most interesting companion, and in her wake Susan and the Ian feel like they're simply splashing about in the shallow end. When Barbara goes head to head with the Doctor, it's the first example of a scene that has now become formulaic to the series. A human wants to change the more barbaric aspects of history (in this case human sacrifice), but the Doctor can't allow it to happen. You can almost hear the 'tink' as Lucarotti's shovel strikes gold in this scene.
But it's 1963, so all that shimmers is most definitely not gold, and The Aztecs marks the first story where The Doctor has a romantic interest. Yes, you heard me. He had a romantic interest in The Aztecs in 1963. When you realise this, it's easy to understand Moffat's defence of his 'sexy' Doctor. He does have a drive, after all. He's most definitely not asexual. Adding even more credence to Moffat's interpretation of the Doctor is his reaction to the woman whom he finds attractive in The Aztecs. He is like a child: curious, warm, bumbling, clumsy. Little changes in the following 900 years, apparently.
At just four parts, The Aztecs is a shorter story and better for it. It's not a stand out, but it possesses enough quirky interest to be a must-watch for most Whovians.