I watched this for the first time in a while tonight (and here I should mention that I'm specifically referring to the half-hour episode "An Unearthly Child", and not the four-part serial that takes its name from that first title). It still astonishes me what an amazing piece of television it is. Perhaps it's the fact that it was remounted after the famous failed pilot, but it really feels very confident in what it's trying to achieve.
It's a glorious little mystery, the sort of thing you can easily imagine hooking viewers of any era--the strange girl who lives in a junkyard and knows more than her teachers, alien and alienated and yet delighting in the quirks and trivialities of "modern" life. And when her teachers investigate, well...isn't that first scene with Hartnell wonderful? Doesn't he come off as delightfully rude and mischievous, sinister and sarcastic and puckish all at once? The energy between Russell and Hartnell immediately gives the story a frission of tension that really doesn't go away until well into Russell's tenure on the show, and it's a trick the New Series has almost completely missed--all the New Series companions have been willing daytrippers, able to go home whenever they pleased. I've always felt that the Classic Series, especially these early episodes, capture a bit of the double-edged sword of traveling in the TARDIS. You can't get off this ride until it comes to a complete stop.
There are clearly story ideas from this episode that were abandoned, and I don't just mean Susan's existence...the Doctor and Susan portray themselves more like exiled royalty than fugitive thieves. And of course, nobody involved in the production had any idea where it might lead, and certainly not that it would lead to fifty years of adventures. But they certainly built a solid foundation for future stories here.