I have often wondered ...
Well, I can tell you. Six burly operatives in black suits and dark glasses descend upon you, hood and cuff you and you wake up 18 hours later in Azkaban.
No, really .... absolutely nothing! I know, because I have done it. The first time I used ESTA I was terrified, because I changed flight and carrier between registering and travelling. Nothing happened. It was not even commented on. Thereafter, I have (inadvertently) used one ppt to register and another to travel. Again, nothing happens. I've also lost count of the number of I-94 stubs that I have in my ppt that were simply not collected from me on exiting the US.
So, my conclusion is that this silly ESTA system is nothing more than a massive data-mining exercise by the CIA and has nothing whatever to do with Homeland Security. I don't know if the original concerns expressed in this thread are valid or not, but it seems to me that there must be a mountain of data out there that is simply so mind-bogglingly large that it is utterly useless for any conceivable security purpose. Has anyone noticed, by the way, that the visa waiver form has now changed to require telephone numbers and email addresses - what the hell for? Completely redundant data to add to the pile. Anyone can buy a SIM card off the street and change it tomorrow, or open and close an email address. Pure application of Parkinson's Law to an overmanned, overfunded and overstaffed bureaucracy.