What makes a reboot a reboot, and a retcon a retcon — and what makes a show one, and not the other?
In response to this post on the major franchises that haven't yet been rebooted, a discussion began about reboots and retcons, the ways they overlap, and they ways they're different:
The Star Wars prequels are arguably reboots. Midi-chlorians, new characters (and new versions of characters), a different tone and execution, Midi-chlorians (it's worth repeating), lack of world continuity with the first three films, etc., etc. This is complicated by the "making it up as they went along" nature of the series that created discontinuities from the very beginning, however. (And the fact that the vast, vast majority of the material in the franchise is part of the expanded universe.)
Also, the Doctor Who US TV movie sure seems like a reboot; the new series is, in some sense, a reboot of that reboot, back in line (to some degree) with the original show continuity. The TV movie rewrote the character and tone of the show, which seems like a major part of what makes a reboot- the only real continuity was provided by McCoy and the outside of the TARDIS. The fact that the show isn't big on continuity to begin with makes the dividing line between "reboot" and "continuation, but ignoring a lot of what happened before" to be rather fuzzy.
Star Wars is more of a retcon than anything else. The midichlorians are a classic case of an author waving a magical continuity wand and saying, "There, I never explained how the Force worked or where it comes from, so nothing was contradicted!" You may think they're a dumb idea (and I agree), but they don't invalidate anything in the original movies, even though nobody mentions them ever again, or Naboo, or the Sith, the Trade Federation, etc.
The Doctor Who TV movie — which started out as a pilot for a new series — was technically a revival/continuation of the BBC show, though again, it didn't contradict or invalidate anything we'd seen or heard before. There's that "half-human on my mother's side" comment the Doctor makes, but that could be interpreted in any number of ways. Or just ignored and forgotten, which seems to be the case. Had the TV show gotten a series order, it would have played havoc with the Doctor's origins, but that didn't happen, and the Eighth Doctor of the audio adventures ended up getting officially recouped into canon without all that excess baggage.
The interesting thing about the Davies/Eccleston version is that it really does start out feeling like a reboot — there aren't any explicit references to the old show, at first, and even when they do turn up they're fleeting and non-specific: Nestenes, Daleks, a Cyberman head, or UNIT (but not the Brigadier). Someone watching the show in 2005 could easily assume that the writers were simply using elements of the old program in order to build up the universe, without referring specifically to the original stories, a la Marvel's Ultimate line. It really wasn't until "School Reunion" in the second season that we had a definitive link to the original series, with Sarah Jane and K-9. You could argue that the original Who and the current version are two fundamentally different shows, and they are, but they are about the same character and the same universe, so nothing has really been rebooted. (One could argue also that the show's continuity is so loose and porous by nature that it doesn't really require reboots.)
So, what do you think? What's the distinction between a reboot and a retcon? And what are some of the shows that illustrate either perfectly?