For those of you who weren't regular viewers of Star Trek: Voyager (heck, I was one of 'em, just catching up via reruns), in the pilot episode, when Voyager got thrown into the Delta Quadrant, they didn't have a medical officer on board, so Captain Janeway activated the Emergency Medical Hologram.
Now, the EMH was a solid hologram, like the holograms on the Holodeck (which are either solid light constructs, or something else entirely). Strangely, the one saved in Voyager (and so far as we know, it's the only model that exists, assuming any other ship has an EMH program) is balding and not a little... well... whiny and bitchy, at least at first. Yes, the EMH had a personality, even from the beginning.
Before I get into talking about the Doctor himself, let me pose this question: Why wasn't the Emergency Medical Hologram based on a specific Starfleet Medical Officer, and then given that person's name as an identifier (along with their personality)? After all, in Next Generation, at least, we've seen that with minimal input, the Holodeck can generate holograms of people that act pretty much like the real thing... if someone volunteered to be the template for the EMH doctor, their own personality could have been completely encoded into it.
Now, for all I know, this was what basically happened... although I've yet to see an episode that specifically says that's the case.
Anyway, on to the Doctor. The Doctor's got plenty of personality from the get-go, and since Voyager can't get a living medical officer to take his place, they continue using the Doctor... and eventually, they give him a device that allows him to participate on Away Team missions (which, bizarrely, is attached to his arm, so it's generating a solid light hologram to hold itself in place!).
The Doctor also evolves beyond his programming during the course of the show, since he has to operate on a full-time basis.
Now, if the only sci-fi TV shows you watch are those created in the USA, a solid light hologram character who evolves beyond his original programming might seem to be a pretty clever concept.
Except it's kind of been done before. Sort of.
I've occasionally talked about Red Dwarf, one of my favorite British TV shows, a sci-fi sitcom. There are two characters on Red Dwarf that, combined, basically give you the Doctor on Voyager.
First up is Arnold Judas Rimmer. When the series starts, he's alive, but by the end of the first episode, he (and the rest of the Red Dwarf crew, save one) is dead, dead, dead. His slobby roommate, Dave Lister, is the only human survivor of the disaster, and the ship's computer, Holly, determines that in order to keep Lister from going insane, Rimmer must be brought back in hologram form (the Red Dwarf can maintain one hologram crew member at a time). Eventually, Rimmer gets a "light bee" that flies around inside him, generating his hologram form, and later on, he even gets the technology to become solid light.
Like the Doctor, Rimmer spendt a lot of time being whiny and bitchy... and bemoaning his fate (although to be honest, the Doctor is a nicer person overall, and is actually competent at his job). Oh, and just to be complete, eventually Rimmer takes the place of himself in an alternate reality, and then nanites bring him and the rest of the Red Dwarf crew back to life. (Don't ask)
Then there's Kryten, who was introduced later in the series. Kryten is a service android, built to be the ultimate butler/housemaid. He takes much pleasure in cleaning and doing menial chores, but Lister helps him eventually break his programming and be more independent and human.
So, like I was indicating, combine the two, and you've got the Doctor.
But you know... there is one other thing about the Doctor that bugs the crap out of me... and that is just how well-rounded the Doctor becomes, to the point where he even asks for a name (i.e., he seeks an identity of his own... talk about cognito ergo sum!)! He grows and develops as the show goes on... and there's nowhere mentioned that Voyager's got special computer programming in place that's way beyond what, say, the Enterprise-D had.
On the other hand, the Enterprise-D has Data. Data is a self-aware android, of course, who is always striving to be more human... and spends years in that process before getting the emotion chip that allows him to experience the range of human emotions.
Much was made about how big a deal Data was, how advanced his neural net was and so forth... except that the more Voyager episodes I see, the more it becomes obvious to me that the Doctor is at least as advanced an artificial intelligence as Data, if not more so (because the Doctor did experience emotions from the get-go).
This strikes me as one of those things the later Star Trek series did that sounded like a very neat and cool idea, but wasn't thought through all the way.