During its original run and well into syndication, I couldn't stand Quantum Leap — the "rules" governing Sam's time travel made no sense to me. "He's bouncing around from time period to time period, and he's stuck in each life until he makes things... nice??" One weekend, the Sci Fi Channel ran a marathon of the final episodes of various shows, and I happened to catch the finale of QL, which reveals that the controlling impulse behind Sam's "leaping" is his own subconscious drive to make the world a better place, one life at a time — and I then proceeded to catch the show five days a week for the next few months. — Your Obedient Serpent
The other thing that I realized (then Googled and realized to my disappointment I wasn't the first) was that, especially clear during the last season, he was realigning the "Ziggy Version" of history to match our version and that after those "major" events in modern history were lined up, he was done except for one last thing (having Beth wait for Al) which completed the alignment and therefore returned to his "Ziggy History" timeline and disappeared from our timeline (thus ending the series) — Gaidin BDJ
In "Star Light, Star Bright," Sam is hypnotized, and on tape, gives away the secret access code name and numbers regarding top secret information about Project Quantum Leap. An episode later, we meet an evil leaper and hologram. Clearly, someone else found the tape, accessed PQL's files, and set up their own project for personal gain!
And thanks to the events of "Shock Theater" and "The Leap Back," one of the Project's handlinks is left stranded in the 1940s and unaccounted for. It won't work for another 50 years, but it's completely intact and undamaged. Between that tape and a piece of technology that could be reverse-engineered...
That explains why the evil leaper organization does everything exactly the same with different sound effects; hologram technology, handlink, etc. Plus, Zoe was aware of just how long the window was before a successful retrieval could be performed, which means the evil organization may have improved on PQL's math.
Throughout the series, Sam displays a reluctance to engage in any sort of relations with women, a fact that Al laments constantly. Near the end of the show's run, it is revealed that Sam is married, and suddenly his hesitation can be interpreted as a subconscious reaction to the possibility of cheating on his wife. It also gives a more unfortunate implication to the times when he does it anyway, though.
Donna states that she does understand, knowing that he can't remember her and is doing it to improve the lives of those involved.
In "The Leap Back," when Al drops the letter with the override code (and the $100) to the law firm, with instructions to deliver it on a specific date in the future (a la Back to the Future), the imaging chamber door immediately. This is because, as soon as Al dropped it in, there was no way for him to retrieve it. Hence, the "outside" time was when it was delivered, according to Gooshie, that very day.
Occasionally, history changes to avoid a disastrous marriage. This was even the point of at least two episodes. The problem? Some of the marriages resulted in children but now, those children will never have a chance to live. Sam and Al are essentially murderers and never show any remorse about it. In one episode Al even gleefully proclaims something along the lines of "Now she won't end up having a couple of screwballs!" Suddenly, Sam and Al don't seem like such nice guys.
It's not murder if you prevent someone from existing but, yes, that is pretty horrifying. There are presumably new kids who exist who live better and happier lives than the old timeline's but saying that the happiness of the parents is worth more than the existence of the children...It's a problem.
I always thought of it like The Inquisitor from Red Dwarf. Instead of children being born who would do nothing with their lives, we get children who never had a chance to exist being born, children who might contribute something to society. And considering that Ziggy never reported that their children did anything (as the OP pointed out, they were a couple of screwballs), then we must assume that this was the case in the other occasions when Sam broke up a marriage.
Plus given the existence of a seemingly benevolent God in this setting, chances are that while the kids as we know them were Ret Gone, they exist in some sense as different kids.
It's been accepted by fans for years that Sam's changes to history were slowly turning his timeline into ours. The problem? His timeline was far more advanced than ours, not even counting time travel. His efforts have resulted in progress slowing down!