Believe it or not, the biggest problem about 'The Screaming Jungle' isn't the production values.
No, seriously. I mean, yes, Terry Nation was asking Ray Cusick to do stuff that most series wouldn't really be able to do effectively until the era of CGI, and certainly no series would be able to do effectively on the kind of budget Cusick was working with. So we get limp fake plants dangled from just off-screen on Ian and Barbara's heads, and even though they do their best to respond to this as an urgent and credible threat, it's still hard not to feel as though they're barely repressing giggles. But that's not the problem with 'The Whispering Jungle'. Other episodes have looked just as cheap, if not cheaper.
No, the problem with 'The Whispering Jungle' is that there's not much there once you get past the production values. The previous two episodes have both had interesting hooks to grab viewers--the initial episode had the creepy, silent Voord assault, and the second episode had the surreal and paranoiac brainwashing sequences of the Brains of Morphoton. Once the audience's memory did its usual job of replacing the weak special effects with a bit of budget-stretching imagination, they were interesting uses of the science-fiction tropes they embodied.
But 'The Whispering Jungle' is the point where the imagination, as well as the money, runs out. This isn't Nation's fault, really; again, this is a story written on incredibly short notice to fill a gap left by a script that fell through. Everyone is doing their best under incredibly trying conditions. It's just that here, everyone's best isn't quite good enough.
Again, as with the previous two episodes, the concepts aren't bad; a creepy jungle where the plants have turned aggressive and malevolent, and are eroding the planet to death with hateful overgrowth is an idea that will be used to great effect later in the series. A temple filled with deathtraps, and a paranoid old man hiding in the center of them unable to let go of his fear long enough to realize that these are the people he's been waiting for all this time? That's some gripping stuff. But in practical terms, all that winds up coming into play only in the last few minutes. There's a tiny bit at the beginning where someone drags a creeper across Susan's legs, and the bit at the end shows some of the promise that could have been, but the whole thing lacks pace. The plants should have been a menace from the beginning, a relentless pursuer that drives Ian and Barbara headlong into the deathtraps. The traps themselves should have been more menacing--a bit of a change in the direction here would have gone a long way. It's understandable that Cusick didn't have much to work with, but better pacing and a few changes in lighting and camera angles might have mitigated the flaws.
This isn't to say that the episode doesn't have its charms...there's a fascinating moment near the beginning where Susan explains that she decided to go on ahead of the others because she didn't like saying goodbye to her grandfather. Given that Nation also wrote 'The Dalek Invasion of Earth', Susan's departure story that ends with the Doctor going on without her, it's interesting to think of this as a little piece of foreshadowing. It's also a good example of a character point noted by Wood and Miles in 'About Time'...as a product of a technophiliac society, Susan tends to get unnerved far quicker in natural environments than she does in alien spaceships and space stations. Carole Ann Ford has gone on the record about her frustrations with the hysteria she was forced to depict on multiple occasions, but they did seem to do their best in making it a character trait beyond simply "damsel in distress".
The moments like this are a bit few and far between in this episode, which is a shame given the potential that was wasted. But they are there. The money may run out, the time may run out, but Doctor Who always has at its core a concept that can sustain even the most exhausted of writers. And this episode, that describes Terry Nation perfectly.