There was - and possibly still is, for all I know - sort of a received wisdom in Doctor Who fandom, and it goes something like this: Doctor Who starts off with a really really great episode, followed by three episodes of guff with cavemen before getting to the really exciting stuff with the Daleks.
I'm not necessarily about to say this received wisdom is wrong, exactly. But it is a bit unfair, and it's rooted (I think) in the problem of not seeing it as the original audience did. It's easy now - particularly as, for many people, their first exposure to a version of An Unearthly Child may have come in the form of the novelisation Doctor Who in an Exciting Adventure with the Daleks or the film Doctor Who and the Daleks - to assume that the cavemen story was at best a sidestep, at worst a mis-step; but for the first audience, there was no real clue of what was to come - this was what Doctor Who did - it took two teachers off in a ship disguised as a police box and plonked them down in prehistoric earth.
That said, this episode isn't without its flaws - chief among which being the decision to have the caveman actors speak in a sort of slow, zombiefied way to suggest that "primitive" equates to "a bit thick". Horg is the worst offender. But on the other hand, I'm sure I read that the original intention was for the cavemen to only communicate in grunts, so perhaps we should be grateful for what we got.
Also, Ian's refusal to believe that the TARDIS has taken them back in time is a tad overplayed - he comes across in this episode as a bit of a Dana Scully.
But those gripes aside, this is a neat, compact episode, telling an interesting - if narratively rather linear - tale of what happens if a tribe of cavemen seeking the secret of fire suddenly come across a strange old man with a box of matches (which he subsequently loses). The cavemen politics is interesting, setting up the conflicts quite neatly, and the time travellers' section of the narrative helps to further establish the characters of the Doctor and Susan, as well as the nature of their ship.
Oh, one more quibble - The Cave of Skulls seems a rather dopey choice of title for an episode in which the cave in question only appears in the last few minutes.
And an observation - without wishing to be crude, is it just me, or do Za's attempts to make fire by (apparently) rubbing a bone in his hands over a pile of sticks look suspiciously like he's pleasuring himself? Seriously...