Boy, have they been wanting to do this one for a while. You can see it in the carefully structured reveal--this is something they've been thinking about for ages. The journey through the stone labyrinth, the discovery of the giant worms and ants (the one weakness to my mind--Barbara calls it a "snake" when it's clearly an earthworm, which makes you wonder a bit too soon what else they're not noticing) and finally the revelation of the giant matches and the crazy-paving. It's all set up so deliberately that you can almost hear the entire production team in the background going, "Wait for it, wait for it..."
But that's really how a first episode should work in Doctor Who. The classic series has always been about exploring spaces; Part One of any given story is usually centered on the Doctor learning about the structures of the new environment he's landed in, whether social or physical or biological. Here, it's not just about the discovery that he and his companions are little and tiny and wee and adorable--it's about the way that they gain understanding of the entire ecology of their new world.
Which is, of course, in deadly peril thanks to Forester's DN6. It's both wonderful and sad that this little eco-fable hasn't aged very much at all in the five decades since it originally aired; to this day, there are still people trying to turn back the march of regulations so that they can spray DDT all over the landscape, ecosystem be damned. Forester isn't really any different from Tom DeLay, except that DeLay tried to crush inconvenient scientists with political power instead of just shooting them. (Mind you, some of that may have been that his enemies didn't come visit him without telling anyone where they were going, giving a convenient excuse for being missing for several days, and then turning their backs for a while just to see what happens.)
(Well, I mean, I assume they didn't. Maybe they did and he was just better at hiding the bodies.)
The point is, it's okay that this is an episode that's mostly just the Doctor and his companions wandering around and exploring, because that's how the series creates worlds for us to believe in. Even if those worlds are right outside our door.