Raiders had good plot. It just effortlessly and smoothly inserted it between the cool set pieces, and acknoledged that people weren't coming to see this for the plot.

The Mummy (the first one of the modern redo of that) does Indianna Jones better than Temple of Doom or Crystal Skull because it understands this.

It seems that George Lucas can come up with great films when he first tries a concept, but he then tinkers and messes with it too much, and loses the purity and simplicty that was the things strength.