This story was tied for second (with two others) for the Sturgeon Award for Short Fiction; I have long since ceased to caper about chanting, "We're One Third Of Number Two!" It also made it into the Gardner/Strahan Best of the Year anthology. So it comes highly recommended by people who read an awful lot.
This was sort of a trial canter for several ideas that I know I'll be back to later (some of which I've been to a few times before): the mutability of memory, the awkwardness of remembering things that other people don't (or say they don't), the genuine alien-ness of people with various socially-isolating brain syndromes, are all in one complex. In particular, in most stories in which time travelers alter the past, either some time travelers are immune, or everyone's memory changes instantly; I wondered what it might be like for a mind to feel the world ghosting in over it, maybe taking months or years (an idea I sort of played with in Finity and am sure I will play with again).
Another one has been with me ever since I noticed what the copywriter had come up with for the cover of an S.M.Stirling novel: "Think about history. Imagine it's worse." That led me to an idea I'm still playing with: how much worse would history have to be for us to declare a do-over or a scrape-and-pitch, i.e. just decide, well, that was all wrong, let's do something else? Pick your favorite bit of horror out of the last few centuries, say, and imagine sending back a force whose job was to prevent or mitigate it. If it's big enough, most of us would either cease to exist or cease to exist as ourselves; but if it's bad enough, might it be worth it? And how bad and big, and what about all the people who are just blameless byproducts?
Another idea: "live and let live" is an old idea but its prevalence is very much modern. Ages ago I ran across what seems a truthful comment: if you brought a European forward from 1200 A.D. to the present, the most inexplicable thing about all our greatly expanded powers to him would be that we hadn't declared a crusade and use the atom bomb to reclaim the Holy Sepulchre. It's probably--well, definitely--a good thing that our ancestors didn't have all our capabilities; you can construct your nightmares here about what a society with airplanes, electric fences, behavioral conditioning, effective psychopharmacology, and poison gas might have done in the way of slavery or genocide, back when those were done with virtually no compunction.
What sort of modern world might grow in a society that poisonous?
And what if we were descended from them? What if we could then reverse things so we would only, in that revised world, have been as wicked as we actually have been? How many never-weres would be a fair price to move from deep to less-deep evil?