Since I was a kid Indiana Jones has been my icon for real manliness much as John Wayne was for my father. So when last year's Indiana Jones and the Crystal Skull sucked it, I took it personally. I only mention this now, months later, because I stumbled across(t) an entry on the aptly titled blog Temple of the Seven Golden Camels where the author gives what I feel is the best reason for the failure of said flick.
My contention was that all of the sequels suffered not just from George Lucas' goofiness, but that they didn't have the one thing that the original did: that Raiders of the Lost Ark introduced us to a world that was seemingly like ours and it wasn't until the end that we realize that things like the Ark of the Covenant really were radios for talking to God and such. The other films begin with the premise that these kind of objects already exist and so we expect it.
The aforementioned blog posting agrees with my postulation somewhat, but insists that the real reason is that Indy has gone from a morally ambiguous character who grows in the first film to a flawless and boring hero in the others. When Belloq says to Indy that it would only take a push to make our hero more like the villain, Indy gets pissed because he knows it's true. It's not until he witnesses the power of the Ark that he realizes the power of the artifacts he hunts and their value.
Anyway, aside from that, the last film was a steaming pile of poop, it's only saving grace was the casting of Karen Allen. Here's my depiction of everyone's favorite archaeologist.