No Nazis this time, but the Soviets. lazs is probably wetting himself in glee right now.
Maverick has it pretty much right-on. Indiana Jones has never been known for plot and story. It's a pure escapist action, the plot only being a loose thread to string together the action sequences. Crystal Skull was well-paced, and I haven't laughed so hard at the movies EVER. This was vintage Indy from start to finish. The only two moments I really cringed during was the Lucasian overuse of CGI during the climax, and Shia with the monkeys (I won't spoil it for those of you who haven't seen it yet). A lot of critics blasted Blanchette's Russian accent, but I think it may have been INTENTIONALLY bad because it fit just perfectly with the 50s B-movie atmosphere, and the franchise's own penchant for poking fun at itself (which Indy targets very early on).
Oh, and Crystal Skull has, far and away, the BEST snake gag in the series.
Also, if you compare the actual CONTENT of the reviews on Rotten Tomatoes, you'll actually find that the reviews for Crystal Skull are NOT that far off from the other movies in the series. The way movies are reviewed has changed in the last 20 years and gotten lot stiffer than it was in the past.