I can't say that I knew right off the bat that there was anything was wrong with the PT. I was a kid when I saw the first two prequels, so I wasn't quite able to see the faults of the films at that time. From the beginning I felt that the PT was inferior to the OT, though I didn't see them as bad until much later.
I first saw TPM about a year after I watched the OOT for the first time on video. I never saw it in theatres, but I had read a few novelizations (both junior and adult). I frankly found the novelizations dull when I read them, so when I did see the film I wasn't expecting anything groundbreaking.
In the end I did end up liking TPM once I saw it - much more than its novelizations. It wasn't as good as the OT, I thought, but still decent.
AOTC didn't impress me nearly as much. I didn't like the overuse of CGI, and thought the acting wasn't so great (I had seen Hayden before in over work, so I kinda knew what to expect from him). At the end I was like "So that's how Anakin lost his hand ... so Boba Fett's a clone ... so the Clone War clones were all cloned from some lame bounty hunter ... uh ... okay".
My distaste for AOTC only grew with subsequent viewings, but I didn't hate the film at that point. It was a Star Wars film, and even the least impressive entry in the series had to be okay, right?
I then later read the ROTS novelization ... and I thought "So that's how Anakin turned? That's how the Jedi were wiped out? What, Padmé died? So Obi-Wan didn't believe there was any good in Anakin after he fell after all? Obviously this isn't how things were originally supposed to unfold, and not how I would have done it, but overall a satisfactory conclusion to the saga".
Then I saw the film itself ... and I knew right away that it was worse than the book that was based on it. Worse acting and dialogue than ever before, and even more overuse of CGI. All the faults I found and accepted in the novelization were amplified in the film, and made utterly unbearable.
It was then that I knew something was wrong, but I still managed to convince myself it was a good movie. "Beyond the CGI and bad dialogue it is still a Star Wars film", I thought, "I still feels like a Star Wars film".
It wasn't until I finally got a computer and Internet access that I came to hate the PT, all of it. Prior to that point I had never had regular access to the Internet, and had been sheltered from most of the diverse views the fans held (frankly, up til then I had assumed that pretty much every fan accepted anything SW).
After learning that many fans were distressed by the changes made to the OT and the discrepancies created by the PT I was able to put two-and-two together, and my own dissatisfactions boiled over into outright hatred.
This is my first post, BTW. Sorry if I rambled on there a bit ... =)