I watched Star Wars Episodes I, II, and III recently, for only the second time. I've problem seen the other three, the original three, the good three, a hundred times or more (I watched them a lot as a kid).
When fans of the originals complain about how bad the prequels are, one response you hear sometimes is, "Look, the originals were just as bad, you just remember them fondly because you were a kid. They are all completely absurd space operas with lousy dialog and naive kidsy themes. And you complain about Jar Jar -- look at the freakin' Ewoks!" I've never felt this complaint was quite right, but I've also had some trouble refuting it in the past.
One thing I noticed on the first viewing, and this was while watching The Phantom Menace in the theater when it first came out, was that the CGI they used in the new ones just looks too "clean". One of the lovely things about the originals was that the highly detailed models Lucas and his team built just looked so gloriously dirty. They actually looked like something that had been traveling through deep space for a long time. In contrast, the ships and robots in The Phantom Menace just looked like run of the mill modern sci-fi. (I suppose one could justify this on the grounds that the Old Republic had better, more well-maintained stuff; but this reeks of Star Wars apologetics, and I'm not having it)
However, on the second viewing, I think this was a minor problem at best, and really a subset of a much bigger problem. So without further ado, here is where I think Lucas made two very simple missteps that severely tarnished the prequels and prevented them from having that fantastic charm of the originals.
So let's talk about Jar Jar. No discussion of what's wrong with Episodes I and II can be complete without discussing the much-maligned Senator Binks. As I stated before, some Jar Jar defenders/fanboy critics have compared Jar Jar to the Ewoks, saying both are equally comic, equally silly, had equally annoying voices, and that both are obviously designed to appeal to kids rather than adults. Fair enough.
And yet, the Ewoks don't piss me off nearly as much, and, though I realize this is hopelessly subjective and I have no real basis to assert this, I just can't make myself believe that the reason is because I grew up with the Ewoks. Flight of the Navigator seemed like a grand adventure to me as a kid, and was also one of my favorite movies, but today I see it for the corny trash that it is (though I do think some of the sound editing, particularly in the early scenes just before the protagonist discovers the spaceship, is masterful). Why did the Ewoks age so much better?
I think I know why: The Ewoks didn't speak English. In fact, they weren't even subtitled. Sure, when Wicket screeches "Beecha-wawa!" after almost being hit by a Stormtrooper's blaster, it's just as grating as anything Jar Jar says -- but it feels like an alien who just happens to have a high-pitched voice, because of the Ewoks' inscrutability: an inscrutability that would have been impossible to pull off if they had spoken English. In contrast, when Jar Jar says, "Meesa so scared!", it sounds like a damn cartoon character.
If you compare both trilogies, this is very consistent. Virtually every alien in Episodes I, II, and III speaks English, whereas in the originals most of them do not (and many of them are not subtitled). Hell, R2D2 and Chewbacca were main characters through all three movies, and neither of them ever said a word that was directly comprehensible to the audience. (As a brief side note, Ebert's review of The Empire Strikes Back notes -- correctly in my opinion -- that the film's biggest flaw is Chewbacca's incessant mournful cries. They work to express the intended emotion, but they just get so repetitive. However, that was a problem with execution, and a minor one at that. Just imagine how bad it would have been if Chewbacca had the same voice, but talked in pidgin English! Perish the thought...)
Think of the scene with Greedo in Episode IV. Nobody is going to call that scene kidsy -- Han Solo murders him with a gun concealed under the table, for chrissakes! But think about it: Greedo had a pretty funny-sounding voice, didn't he? Imagine if Greedo had that gurgly, almost trilly voice, but spoke English instead of Rodian. Granted, it wouldn't have quite been a disaster of Jar Jarian portions, but it would have significantly undermined the tension in the scene. (Another brief digression: In the updated version released in the 90s, why oh why did they have to make Greedo fire first? It's so freakin' corny... From a realism perspective, how the hell would he miss at that range? And from a character development perspective, it sort of undermines the early establishment of Han Solo as a "scoundrel". Oy...)
In any case, having your aliens speak their alien language allows you to get away with a lot more, without having it come across as cartoonish. I'm sure Lucas felt that having the characters speak English improved pacing and made the movies more kid-friendly by obviating the need for subtitles -- but I was probably seven or so when I first saw the original trilogy, and I still was enchanted by it. Big mistake.
On a side note, I have no particular opinion on the racial aspects of the anti-Jar Jar criticism. I am certain it was not intentional. But I absolutely see how people perceive it that way -- though on the other hand, my wife does not see it at all. I'm not sure if it rises to the level of being racially insensitive or not. As I said, I have no strong opinion on that issue.
Moving on... The other big problem with the prequel trilogy, particularly Episode I, is more subtle, but possibly more pernicious. You see, one of the reasons the original trilogy worked so well despite a number of absolutely preposterous conceits (a freaking laser sword being the preferred weapon of the Jedis? When guns are available??? Uh huh...) is that the entire Star Wars universe exists in a vacuum. You can make yourself believe it's in "a galaxy far, far away", because there is a total absence of pop culture references. Those concepts which overlap with our world tend to have a historical mystique to them ("Imperial senate", "knights", etc.) which contributes to the other-worldly flavor rather than detracting from it.
Now think back to the pod racing scene in The Phantom Menace. Intended to be the most stunning set piece of the movie (they had a freaking video game based solely on that scene timed to coincide with the release of the film!), it was marred by the corny two-headed announcer(s). Out of context, there's nothing so bad about that gag. It was like something straight out of a Pixar movie, and you know what, all the Pixar movies are pretty good. I'm sure kids found the announcer(s) to be pretty funny, and while the jokes weren't exactly scintillating humor, the dialog was passable as it goes.
But the reason that gag "worked" is because it was a parody of the standard two-person commentator team used in most US sporting events, play-by-play and color analyst. It was a direct pop culture reference. So what the fuck? Does this "galaxy far, far away" get American broadcast television? From the future? No, of course not, it was just a bad choice on the part of the scriptwriters, but nonetheless it degraded the illusion of otherness that pervades the original films.
Though the two-headed announcer(s) were the most egregious example, such problems pervade Episode I, and are present to a lesser extent in Attack of the Clones as well. (I feel Revenge of the Sith finally broke this trend, and is one reason why many fans feel that is the best of the prequel trilogy) I mentioned earlier that the ships look too clean; they also look too much like modern conceptions of sci-fi. Compare Qui-Gon Jinn's ship in Episode I to the Millenium Falcon, both pictured below. If you had never seen any Star Wars movies, and I told you that one of these was thought-up by humans and another was built by aliens, what would your guess be? I thought so. Most of the other ships from the original trilogy are less alien than the good ol' Falcon, but they still don't look like modern fighter jets. (Which brings me to a minor realism niggle: All of the ships in the prequel trilogy are highly aerodynamic, which doesn't matter a damn in space. Many of the ships from the original trilogy are far less so.)
I think there are other examples that are more subtle, and I won't attempt to list all of them. I'd have to watch the movies again in order to catch many more anyway, and I don't expect to be doing that again until my eldest son is old enough to dig Star Wars. But I'm telling you, I think this sort of problem is what wrecked the prequel trilogy, or at least the first two episodes. For all the corny dialog and stilted acting and baldly implausible ideas in the original trilogy, it felt like something from out of this world -- which made suspension of disbelief that much easier, and made all the crap much easier to swallow. The flaws were partially obscured by a fantastic other-worldly mystique. The prequel trilogy mostly failed to cultivate this atmosphere, and as such all of its warts were laid bare.