My take on it is that 2007 Steven Spielberg was not equipped to make an Indiana Jones movie. The man is immensely talented, and he's got a ton of experience behind him, and from a craftsmanship standpoint Indy4 is excellent, but what an Indiana Jones movie really needs is that endlessly inventive, youthful energy that he brought to the original trilogy. Not that I think the first three movies were shoddily made, but it's obvious that Spielberg's decisions on them were swayed more by "Hey, wouldn't this be cool?!" than anything else, and that's the kind of mentality that you really need to have when making an Indy movie. The first three movies were made by a kid at heart, but this movie was made by someone trying to emulate his inner child rather than truly embodying it. It was like, "Okay well, this is an Indiana Jones movie, so we need a scene like this:" instead of it just coming naturally, or at least that's how it felt on screen. I don't know if it's entirely Spielberg's fault, he really is a different director than he was back then, and you can never entirely go home again. In a sense, the Spielberg who made Indy4 didn't know how to make an Indiana Jones movie, and perhaps to some degree Spielberg himself knew that, which is maybe why he was so reluctant to join the project.
I don't mean to sound so harsh, though. I don't really think Spielberg has "lost it," I just think he needed a chance to find it again. Crystal Skull, despite being the fourth installment in a franchise, felt a lot like growing pains to me, because Spielberg was trying to get back in the groove of this comic book inspired action hero after 20 years of Schindler's List and Saving Private Ryan and his grittier scifi flicks. I think it took the entire production of Indy4 for Spielberg to fully get back into Indy mode, and I'd hate to see that rediscovery wasted, which is why I'm one of the proponents of Indy5. I have some confidence that he could make it great now that he knows what he's doing again. It's true you never forget how to ride a bike, but after twenty years you might need one circle around the block before you can start jumping ramps and popping wheelies again.