4. Playing With Time Travel
Having a main character who grew up with time travel being as ordinary as trees presents a whole lot of cool storytelling possibilities. Each man brought his own ideas and interests to the genre, and the results were awesome. Davies was interested in the magnitude of time travel, in what knowledge of it did to an individual. That's why Rose was nearly overwhelmed by the time vortex, why Eccleston was always so pained. And he was interested in the isolation of time travel, in the distance it put between the Doctor and his friends: in how lonely it made him. The end of "Journey's End" was heartbreaking because the Doctor was left more alone than seemingly ever before. And in "The Waters of Mars," he became so caught up in the magnitude of time and space that he forgot what it was to be human. Finally, when the tenth Doctor did his farewell tour, it was as much about how paradoxically tangential yet essential the Doctor was to the lives of his friends. All because of time travel.
Moffat has a more playful attitude to time travel. He likes to create paradoxical plots, wacky mobius designs where cause and effect are eating each other. And rather than focusing on how the Doctor's time travel affects his persona, he likes to look at how time travel affects the companions. That's why Amy Pond, River Song, and Clara Oswald are all time travellers themselves, Amy suddenly going from a child to an adult to an old dead woman, River Song ricocheting through timelines, Clara coming back again and again.