The opening of "Strangers in Space" (at least, the opening once you get past the adorably awkward scene where the cast recaps all their previous adventures) is really one of the purest examples ever of what Doctor Who does. The Doctor and his companions land in an eerie, deserted, mysterious environment and are immediately presented with a puzzle...and with death. There's always more to things than immediately meet the eye, they frequently find traumatized survivors who provide cryptic clues to the mystery...and almost always, they quickly find that they couldn't leave even if they wanted to.
But at the same time, the very fact that this is the first version of such a perfectly-structured opening means that there's a lot of room for improvement. Maitland and Carol are both strangely detached in their reactions; they don't seem even slightly surprised at a bunch of time-traveling strangers suddenly piling into their control room, and they seem more irritated than terrified by being held prisoner by mind-controlling aliens. It could be that this was an intentional decision on the part of the production team, some sort of side-effect of the mind control, but the TARDIS crew doesn't seem to react to their unusual reactions. Plus, they're acting a bit out of character as well, particularly when the Doctor says no less than three times that there's nothing he can do and they should leave. (Although there is, as a counter-point, the adorable line, "There's not an ounce of curiosity in me. Now, young man, what seems to be happening here?") The net effect is confusing more than tense; you feel like you must have missed a scene somewhere.
Still, the episode is conceptually impressive. It's easy to get sucked into the atmosphere; creepy, mysterious, powerful aliens that can affect not just the protagonists' perceptions but the TARDIS itself have trapped everyone in an endless cycle of artificially induced fear, panic and near-death for unknown purposes. One crew member has already had his mind shattered by the endless torture and wanders the halls like the walking dead (or, to more accurately describe his appearance, like the Walken dead...seriously, if David Tennant and Christopher Walken had a love child, it'd look like Stephen Dartnell, the actor who plays John). This is really creepy and unsettling stuff, and it's no surprise that the series returns to these themes again and again over the ensuing decades.
It's also nice, as an aside, to see mental illness portrayed with a little sensitivity. For all that Carol and Maitland are concerned that John will harm others in his traumatized state, he's really more threatened than threat. Most people with mental illnesses are not violent or dangerous, and it's nice to see Barbara reaching out to help him once she gets over her initial fright. It's a welcome piece of empathy that doesn't diminish the atmosphere of the story.
The main thing that diminishes the atmosphere, sad to say, is the clunky dialogue and overly-mannered performances of the guest cast, along with a shaky performance by Hartnell. This is one of the first times you can really see him struggle with his lines--not just in a few infamous "Billy fluffs", but in a general reading of his lines that suggests he's working hard just to remember them. It's sad, even if at this stage it's more due to the rigors of the production schedule than the health problems that would eventually force him off the series.
At the end, of course, we see the aliens in the flesh. It's a nice reveal, nicer than people give it credit for--the Sensorites won't look so interesting when they're wandering around and chatting amiably, but the lighting and camerawork in this scene makes them look oddly skeletal and menacing. The tone may be off and the menace may not last, but "Strangers in Space" is definitely an important influence on stories yet to be.