>What about antibiotics? That seems like a clear case of changing the world for the better.
At first glance, yes, but then you remember that the universe is not static, but dynamic, constantly evolving, and the introduction of antibiotics, while useful --- miraculous, even --- at first, eventually results in "antibiotic-resistance"[0], and now we have a worse problem than the one we started with.
There is a wonderful essay by Paul Kingsnorth, titled "Dark Ecology"[1] that identifies this pattern as a "progress trap".
This quote from the Tao Te Ching is especially apt:
"Do you think you can take over the universe and improve it?
I do not believe it can be done."
Bacterial infections are no longer a serious threat -- even bubonic plague has been brought under control. It's unlikely that bacteria will ever become fully immune to one or another antibiotic, but if that does happen we've still got penicillin, bacteriophages, and sterilization techniques; bacteria will never again be as dangerous as they were as recently as the late 1800s.
For further clear-cut changes of the world for the better, there's smallpox eradication, the immanent eradication of guinea worms, the extirpation of polio from most of the world, and the development of anaesthesia.
I don't disagree, but bear in mind that 'better' involves a whole bunch of implicit assumptions. Some people think a more austere and purely Darwinian world would be better, and that humanity has gone soft. I don't agree, but try to get in the habit of examining your opinions from radically different perspective to realize how fundamentally subjective they are.
"Better" here just involves the assumptions that pain is worse than not pain, and that human life is intrinsically valuable. I'm fine with making both assumptions -- especially since I've read Timothy Snyder's _Black Earth: the Holocaust as History and Warning_, which explores the thought of the most famous proponent of the mankind-needs-more-Darwinian-selection-pressure alternative. "I disagree with Hitler" isn't exactly compelling evidence for "my opinions are fundamentally subjective."
(I'm also of the opinion that the earth isn't flat, and that both sides of a contradiction can't be true. In those cases, too, I'm comfortable with saying that those who disagree with me are wrong -- and in uninteresting ways, too.)
What about antibiotics? That seems like a clear case of changing the world for the better.