I hate to be the one to start a massive spoiler quote-fest, but...
After thinking about it, my take on the movie:
I understand that movies need us to suspend disbelief, but I think Indy 4 just plain abused this to the point where it lost me as a result.
But that isn't even my biggest gripe with it. The film had piss-poor villains, laughably durable main characters, and bukkaked the audience with CGI.
Let's start with our villains, the Russian KGB. First of all, the cold war was a bunch of silly dick-waving. We all know this now, and as a result, you can't quite group Russians into the same category as Nazis when it comes to Indiana Jones' villains. So we have an alleged Russian "psychic" who never once actually uses her psychic powers, except in one scene in the beginning when she says "You're a hard one to read, Dr. Jones." and then never again. She didn't impress me as a villain whatsoever, and neither did Indy's British friend (forget his name) who kept switching sides in a cliché and predictable manner.
The Indiana Jones series was BUILT upon strong villains which actually strike fear, and you strike fear by getting the audience to relate to something on screen. So we have this crystal skull which grants some power we've never heard of? Not 1/10th as scary as Nazis claiming the holy grail and being immortal.
Which of course begs the question, why weren't these villains Nazis? Why not a group of Nazis trying to use an ancient power to...I don't know, resurrect Hitler? I'm just tossing out ideas, but I couldn't help but be completely unimpressed by what the villain of the movie was suggesting would befall the world if Indy failed.
What killed it for me the most was the fact that, while Indiana Jones is a series built upon clichés, it's absolutely KEY that the actors themselves don't start feeling those clichés because it bleeds through into their performances. Some of Harrison Ford's earliest lines in the movie sound like he's smirking about the fact that he's playing a cliché when he's supposed to be staring down the evil of the KGB.
You can also see this same effect in the "Batman and Robin" movie where, as Ebert eloquently put it, the actors are clearly "winking" at the camera.
I think Spielberg forgot that Indiana Jones movies are built upon characters first and action second, and if you don't have the first part, the second will just be rendered as meaningless fluff, especially when they're riddled with CGI. This is similar to how George Lucas forgot that Star Wars movies were made so successful by the fact that the actors, good or bad, were absolutely dripping with humanity and as such brought audiences to love them.
It's a shame, too, because the first half of the movie which introduced the characters was doing pretty well for itself, then it just devolved into piles of CGI and a horribly lackluster ending.
EDIT: My GF just pointed out that Mila Jovovich's character in "Zoolander" was actually a better and more convincing evil Russian woman than Kate Blanchet's in this movie. Ironically, despite the fact that Zoolander was a comedy, Jovovich played the role seriously.
Mark my words, if this wasn't an Indiana Jones movie, people would be emptying their bladders all over it.