Lots more pages still complaining the turn was too abrupt, should have been gradual over the prequels (as implausible as THAT would be since the Jedi would sense it and take care of the matter). Episodes I and II were setups for the turn in Ep. III. Lucas has said as much. And if a turn is gradual, I'm not sure it's a turn. Wouldn't that be more of a slow slide or something?
And the new one, "Darth Vader is too evil now", to paraphrase. Never thought I'd see people complain that the villain was too evil. Only Star Wars fans... Darth Vader is supposed to be evil. Now that we have proof of what he's really capable of instead of just buying him as a cartoon-character villain people are upset?
You could edit back in Anakin's Jedi killing before the Younglings if you wanted, but really the film as-is is fine. The careful observer will notice the massacre in the Jedi Temple had been going for some time before he took out the children. It wasn't the first thing he did. He LED the troops into the temple. We see that they've been at work wiping out Jedi BEFORE the Anakin/Youngling scene, so you can determine Anakin was killing as well, being that he was the one who LED them in there. He didn't materialize in the Youngling's chamber. Anytime Lucas turns away from connect-the-dot storytelling he gets pounced on for it. He actually let the audience do some thinking instead of showing every little tidbit, and look what's happened.

What did the kid say? "Master Skywalker. There are too many of them. What are we going to do?" That, too, should help the audience realize Anakin didn't just get there, but the massacre has been going on for a while. Do you think he just stood around? I think Lucas was purposely saving the obvious dark-side intoxication for the Mustafar dark-side eyes reveal later. Then the audience can say "Ahhh, what Yoda said is true. Consumed by Darth Vader/the dark side he IS. (And yet there's an inner conflict indicated by the tear.) You'll note you don't get a very good look at Anakin's eyes in the Temple. That's no accident. The editing makes thing happen at a fast pace, but there IS a passage of time and Lucas saves some things for later instead of gratifying the audience up-front. It's clever film-making if you ask me. Why do everything predictably when you can have a little fun with the audience? Maybe he gave audiences too much credit, but I'm not going to knock him for not dumbing it down. Nice to have a mainstream movie play with a little intelligence for a change.
I saw the movie with my dad this weekend. He loved it. This time (third) it was really obvious to me that Anakin made his decision to embrace the dark side fairly early on, well before he pledged his allegiance to Sidious. He talked to Padme after Palpy told him the Darth Plagious story--"I found a way to save you." She promises she won't die in childbirth. "No, I promise you!" It's quite clear that Anakin has already decided he will learn the ways of the dark side to save her, but he does not yet have a teacher, Palpy having not yet revealed himself as a Sith lord. Obi-Wan tells him he's taught him everything he knows which obviously isn't enough in his eyes to save Padme. He needs a NEW TEACHER. Palpy reveals himself; there's a conflict between his allegiances, but the last piece he needed to save Padme falls into place, and he accepts Sidious as his master. It wasn't a snap-decision to turn to the dark side. He had decided earlier. And since there weren't a lot of dark-side masters around to learn from...he becoame Sidious' dark apprentice. It's kind of like Neo's "choice" in Matrix Revolutions. He had already chosen Trinity's life over everyone else's just as Anakin had chosen Padme's over everyone else's.
As far as the OT focusing more on characters and story than the PT, I have to disagree. The PT films are far more personal in nature, and spend far more time with the characters (whether it always pays off is up to debate). In the OT, our easy-to-like, one-line description, heroes are caught up in something bigger. There are huge battles where the BATTLES themselves are the focus. In the prequels, the main characters tend to be the focus while the battle is more in the background. That's why the OT maintains the most spectacular, gripping space and ground battles. The PT is more concerned with staying with the characters and their stories, despite the flashy effects.