They've been trying very hard to do interesting things with this story, but "A Desperate Venture" is really where the flaws of the opening two episodes come back full circle to hit the ending where it lives. "Strangers in Space" and "The Unwilling Warriors" tried to make the Sensorites into figures of menace, and it kind of fell flat because the Sensorites aren't very scary. The next few episodes worked much better because it made the majority of the Sensorites into sympathetic figures, and made the evil Sensorites into cunning plotters who schemed to bring about the Doctor's downfall without direct confrontation.
But here we're back to the Sensorites as figures of menace, and it just doesn't work. When they threaten Carol, it's hard not to wonder why she doesn't just shout loudly at them until they writhe on the floor in agony and then kick them a few times. Or honestly, she could just skip to the kicking--Ilona Rogers is a good head taller than her supposed captors, and it's hard not to watch the scene without wondering why she doesn't just give them a good smack and walk right back out. (Which is more or less how John resolves the sub-plot a few scenes later.)
This leaves the human survivors as the source of any remaining tension (unless you count the supposedly perplexing issue of how they can prove that the City Administrator was responsible based only on Carol's eyewitness testimony and the countless other clues he left behind, or the gripping question of where the hell Barbara got her tan in space). And they can't hold up their end, because they're pathetic. I mean that in the literal sense; they are figures of pathos, gaunt and delusional figures who look like they're about one meal away from dying of starvation without the heroes having to lift a finger to stop them. William Russell and William Hartnell try to salvage the scene by playing it as though they're more concerned for the men's welfare than for their own survival, but this episode is ultimately a series of anti-climaxes.
It could possibly have been salvaged by a different director--the new series would have played up the pathos of the survivors as sad, ultimately broken figures. Or a different take on the survivors could have played up the horror...if they'd been genuinely scary, mad in a sense other than they were, the terrors of being lost in the aqueduct could have felt real and tangible. But at this point, the story tries for pathos and terror and a happy ending all in the same half-hour span, and it fails at pretty much all of them. (Yes, even the happy ending--the Doctor and Susan don't even bother saying goodbye to the First Elder, or John and Carol for that matter. Instead, the Doctor gets a final scene of pointless dickery to set up a next episode that will drop that particular plotline like a hot potato.)
Even with all that, this sets up a lot of tropes that will be used well later. The series comes to love exactly the same sort of reveal that happens here--'The Enemy of the World', for example, uses the subterranean society in the same way that "A Desperate Venture" uses the human survivors. We don't ever get quite so many non-humanoids again as we do here, but the Doctor curing a "plague" and being seen off by the grateful benevolent leader and the cute couple that found true love over the course of four to six episodes is par for the course. Even the secretly-plotting snake in the grass (or worm in the apple) becomes fitted as standard into a lot of episodes. 'The Sensorites', as a whole, is a really good example of the kind of thing Doctor Who does. It's just not exactly an example that's really good.