Legion said:
In Star Wars people can't breathe in outer space for example, and if they did, people would wonder why the hell they can.
To be fair, Star Wars is also futuristic fantasy, not science fiction. It's swords and sorcery, set in space. This, of course, is one of the best examples of the problem Pratchett mentioned.
In theory, science fiction is limited to the realm of things that are likely to be possible based on scientific theory. It's creating fiction based on predictions of where the world might advance to. This definition has one incredibly large flaw though. "Any sufficiently advanced technology is indistinguishable from magic" - Arthur C. Clarke
The Doctor's sonic screwdriver, and a lot of other things from the show, fall into this nebulous area. It, and many of his solutions, might as well be magic. I'm sure that none of the writers are even considering whether or not the things that are happening are based on science. So I can see his point - disregarding scientific feasibility does move the show well away from Sci-Fi.
It doesn't matter - the show is entertaining, and there's nothing at all wrong with Fantasy. Pratchett himself said it too. I'm of the opinion that epically-talented, wildly-successful, knighted authors should get a free pass to nitpick about improper use of genres whenever they want to