One of the generally unexplored themes of the original Doctor Who is immortality. The writers didn't beat this horse, but the viewer knows the Doctor is quantifiably different than most humans in length of lifespan. Yes, he regenerates into a new person, but that person contains basically the same knowledge and interests as the old person. The face may change, the attitude may change, but the Doctor remains the Doctor.
(Rabbit-trail, if a turtle may be permitted a rabbit trail: This concept is similar to the Trill from Star Trek, the Deep Space Nine Trill, not the STNG Trill. While STNG introduced the Trill as a race, and is one of my favorite all time STNG episodes simply on the strength of the script, the Trill were originally more parasitic than symbiotic. I'm glad DS9 took the time to explore that race through Curzon and Jadzia Dax.
Rabbit trail #2: This also provides a wonderful example of character consistency. What immutable character qualities does the Doctor possess that continue despite the change of actor? Concern for humankind, concern for well-being of companions, unquenchable curiosity and a tendency to be flippant in the face of imminent death are a few. Resume topic.)
During the course of the show, we are introduced to Gallifreyans who, because they don't wander about severely damaging themselves every 13 or 26 or 50 episodes, have lived for 1000s of years. The Doctor and his race are nearly immortal. Add time machines to that near immortality, and, well... you have the potential for a very long-lived TV show.
I've admitted my first writing ventures were rewriting TV shows. Sometimes these rewrites were to correct glaring script errors (like bad plots), but sometimes they were simply to insert my own characters into those shows. One of my first character creations was Amy, and she was designed specifically to interact with the Doctor on the subject of immortality.
Originally, Amy was human. She was was picked up by those abducting aliens we hear so much about (can't remember if they're green or gray) and got caught in an intergalactic battle and killed.
Other truly immortal aliens found her mostly intact corpse. Having no previous knowledge of humans and a somewhat flexible moral code, they resurrected her as a kind of experiment and infused her with their own immortality. She has no conscious memory of her human past, and behaves as one of these aliens, but she looks human except for a faded pigmentation (see? easy on the make-up folks should the BBC ever learn of my wonderful creation).
She also provides a perfect thread of continuity for the Doctor's gallivanting ways, since she is a perfectly preserved, completely unchanging being. I even found a spot to insert her in the TV show timeline - right after the Doctor's 4th regeneration when he's still a bit fuzzy from all that molecular rearranging. I mean, if he has the time to mess up Leela's civilization during that five second disappearance of the TARDIS, he could have another single episode adventure, right? Sure he could.
So Amy was born - an immortal, immutable, utterly unique individual. Did I mention she's also photokinetic and mechanically empathic? Yeah, I was young. But I balanced it with emotional autism that causes problems in later adventures, so I forgive myself for the superpower overload.
In the new series, the Doctor's lifespan has become a conspicuous theme. Thanks to the Time War, he is now completely alone in the universe for real. No other Gallifreyans exist. No other creatures understand what it would be like to go on indefinitely while the universe dies around you.
I'm guessing some other youngster realized the waste of melodrama fodder and is now writing for the show as a grown-up.
I hope to continue this train of thought tomorrow. I'm out of time today. heh.