So a movie comes out that becomes an instant hit. It's not an especially well-made film, but it has a couple of things going for it: exciting action sequences, a charismatic lead, and (most importantly) a killer MacGuffin, a filmmaking term for "an object or goal that kicks the film into the third act" (Jim Windolf, "Keys to the Kingdom"). The film instantly achieves a certain classic status. Two popular sequels are released, which combine the same ingredients together into cinematic gold.
However, over the course of the next few decades, the movies become idealized in the American mind. People who had originally been drawn in by the key ingredients begin to see the trilogy of films as something more, as truly great works of art. They raise their children watching them, who, with the innocence of childhood, enjoy the action and the funny lines and the MacGuffin and exalt it possibly higher in their imagination. It achieves true legendary status, unassailable with traditional cinematic critical objections such as believability or good acting.
Then come rumors that one more film is to be made. People are shocked that anyone would even consider adding to perfection, but are curious to see what will happen. And so a fourth film comes out, and is met with...hatred. People despise it. "It fails to live up to the originals," they claim. "It's completely unbelievable. It's stupid. And the MacGuffin makes no sense." Yet the very things they complain about are drawn directly from the first three films. The action is no less absurd, the MacGuffin no more outlandish, the plot no more unrealistic than the first three. But the difference is that the first three are accepted on their own terms, whereas the fourth is being compared to the legendary, inviolable images which most people held in their minds of the originals.
The films, of course, are the Indiana Jones films. The actor, Harrison Ford, mostly carries all four films by himself, only helped by the outrageous action scenes (come on people, is the journey into the "inner sanctum" of the grail any less outlandish than surviving a nuclear blast?) and the supernatural MacGuffins: i.e. the Ark of the Covenant, the Holy Grail, and El Dorado. People claim that the the fourth film is much worse than the original three (or at least, Raiders of the Lost Ark and The Last Crusade, since Temple of Doom gets a bad rep from just about everyone). But they fail to see that, in terms of MacGuffins, the angel of God coming out of the Ark is no less outlandish than El Dorado having been built by space aliens. The only difference is that Raiders was accepted on its own terms for what it was, and then elevated to unassailable mythical status, whereas Kingdom of the Crystal Skull has been compared to that mythical symbol and then despised for the very things that made people love Raiders in the first place.
This may just be a rant, but I hate to see people put down a film like Crystal Skull for such irrational reasons as a hatred of something new that dares to try and live up to something old. When people try to make a film more than it actually is, the film fails miserably. The Indiana Jones films were never meant to be more than a combination of the best elements of 1930's B-action flicks, and they succeeded in that. The new one, since it had been pushed up 20 years into the 1950's, decided to combine that original feel with the best elements of 1950's B-sci-fi flicks, and despite its success at doing so, people hated it for that.
If people could somehow take the original films off the pedestal where they have placed them and compare them rationally to the fourth film, I think they would see that Crystal Skull follows in the Indiana Jones tradition very well. No, it's not a great film, but then, neither were any of the original trilogy. What the films do is capture that little child inside us who wants to be out saving the world, kicking butt, taking names, and finding out what really happened to the Ark, the Grail, and the City of Gold.
So instead of criticizing, sit back and enjoy The Kingdom of the Crystal Skull. It has everything you loved about the originals, if only viewed from the right perspective.
EDIT: Well, it turns out that George Lucas has already said it, and better than I did: "The fans are all upset. They’re always going to be upset. ‘Why did he do it like this? And why didn’t he do it like this?’ They write their own movie, and then, if you don’t do their movie, they get upset about it. So you just have to stand by for the bricks and the custard pies, because they’re going to come flying your way." ("Keys to the Kingdom")