I've always felt that An Unearthly child is a 5 star intro episode into a 2.5 star story - which averages into a good but not outstanding story.

The first episode is as good an intro to Doctor Who as you could get - and sets up the arc on the early shows (and arguably the whole Hartnell era) where the Doctor evolves from being this untrusting, quite crotchety old man to someone that is more accepting of others and generally warm hearted. Ian and Barbara are likely characters who play a big role in establishing key parts of early Doctor Who. The initial scenes in the TARDIS are iconic - and especially interesting to compare to more contemporary 'first time in the TARDIS' scenes.

This contrasts heavily with the caveman story in episodes 2-4. I think that going for a fairly understated intro story makes sense in order to develop the key character traits for the early scenes in a way that a story with a stronger support cast would not do. It does feel at odds with the initial educational aims of Doctor Who - the cavemen aren't exactly historically accurate from the records we have so the historical education merit is questionable.

What I really like is the way that it sets up future elements which shows that they had a longer term arc planned - the Doctor's geiger counter breaking setting up why the crew were unprepared on Skaro down the road; the Doctor being on the brink of committing murder to save his own skin shows that he started out as a much more ruthless character that moderated over time and other elements. The cavemen story has a very simple angle - a leadership battle between Za (son of the former chief) and Kal (an outsider that has great prowess in hunting) over the leadership with the ability to make fire the key point and the TARDIS crew trapped in the middle. It's one of the thinner Doctor Who plots in history and its of no surprise that the series was headed towards cancellation before the Daleks hit; but I think that works somewhat in this context.

Overall: an historical important serial simple for being the first story in the history; but outside that fact and the great first episode there's not a lot to get your teeth into here.